Rosalina & Luma
I love Rosalina's dark red alt (5) for being a very deep color yet still highly saturated, her purple alt (6) for having the dress being complementary to her hair, and her red and white alt (8) is always a treat to see. But for similar reasons that I gave Peach's black and yellow alt the victory, I'm going to say the winner is Rosalina's 7th alt. You have this dark, black dress with bright, saturated yellow accents all around, matching with her hair as well. Bonus points for having the stars on her dress being yellow, making looking at her black dress similar to looking at the actual night sky.
Little Mac
Quick shoutouts to "Almost U.S.A." Mac (4) for being the best Mac alt without the hoodie. It's a really nice mix of blue and red—with the blue being very dark yet saturated—as well as a bit of white thrown in there. However, the best Mac alt is without a doubt Hoodie Mac (6). I love myself a pink and green contrasting color scheme. Both colors are highly saturated, making Mac stand out a lot more when using him in-game. And yeah, I know the hoodie is pink, which represents femininity in graphic design, being the exact opposite of what Mac is supposed to be, but you know what? I think the pink actually makes him look very manly. And I know the other hoodie alt (8) avoids this issue by having the hoodie be green and the gloves be pink, but I'd rather be using the real Little Mac instead of some Wireframe copy.
Greninja
Greninja's alts seem to have fun experimenting with his body and tongue color. In the case of Green Greninja (6), it makes both colors analogous. In the case of Pink Greninja (3) and Gray Greninja (7), it makes them monochromatic. And in the case of Purple Greninja (5), it makes them contrast.
Choosing between the pink alt and the purple alt is absolutely tearing me apart, Lisa. On one hand, the dark purple contrasts with both the green tongue and the yellow accents. On the other hand, the pink alt just makes Greninja nearly entirely monochromatic and does a good job doing so. Also, pink Greninja!
I think in the end I'm going to have to go with the purple alt. I admire the contrast between the body and tongue color, and the dark color of the body goes well with the light color of Greninja's water attacks. The pink alt came really close though.
Palutena
God, do I absolutely love Palutena's 3rd alt. It's yet another godly showing of cyan and purple, but the yellow accents make the color scheme of her dress triadic, and it's absolutely gorgeous to look at up close. Throw some green in there, and have it be complementary to the staff and shield color, and you have yourself a recipe for success. However, I think this alt barely loses out to Palutena's red alt (6). The dark, saturated red tones along her dress are gorgeous and help those gold accents stand out more, but what really sells the alt is how the red dress is complementary to her green hair color. Palutena's green hair always looked good, but the red outfit makes it look even better. I think the cyan and purple alt, while good, throws too many colors in there, all of them being bright and vivid. Whereas the red alt lessens the palette a bit but makes everything look beautiful.
Pac-Man
Mitt gang (8).
Robin
I applaud Green Robin's (3) attempt at an analogous green and yellow color scheme, as I think it works out well. Not only does the green cloak go well with the yellow accents, but it also fits with his yellow hair, Levin sword, and all of his thunder attacks. I also like the white and red Robin (7) for pulling off the color scheme well, but I think it goes a bit wrong with his hair, whose dark color does not look well with the rest of his body. The blue Robin (5) has an outfit that contrasts with the yellow accents, making them stand out as a result (and it even throws a bit of purple in there). Finally, I know the pink Robin (8) has its fans, and I can't deny that it certainly looks pretty.
Despite all of this, I think Red Robin's new look (4), which I will now call Red Robin Requiem, is the best Robin alt. You have a red and yellow color scheme throughout the entire body, creating this really nice analogous blend of warm colors (sometimes the shading of the yellow accents leans a bit more towards orange). I think what really sells this alt is the new hair. Instead of being red like in the last game, it opts for yellow, which still fits within the color scheme of the cloak, but instead of blending in, it pops out at you like all the other yellow accents. It also helps that the hair looks natural instead of looking like it's dyed.
Shulk
All of Shulk's alts are good, but ones that really standout to me are his Melia alt (6), which is a good analogous combination with pink and blue being the main stars of the show; his Sharla alt (5), which I think is very under-appreciated, as the combination of the bright saturated blue and the dark saturated red is freaking fantastic; and his Riki alt (7), an alt that I absolutely love for combining purple with a great saturated yellowy-orange color, plus a few black accents all around. This color scheme is fantastic, and I was really close to picking it. However, while I think it's fitting for someone like Riki, who is a very fun, comic relief character, I don't think it fits Shulk's serious nature. You could argue that Shulk has become a bit of a meme status, but I think the best alt for Shulk is:
Dunban Shulk (3). Black and white are the stars of the show here, and while they're a good monochromatic colors that contrast with each other, there's also a bit of red thrown in there to make things interesting (and for some reason weird yellow accents on the sleeves and shirt). The black color is always tinted red to make the red colors feel more at home, and said red colors help incorporate the Monado's red color into the alt. It gives Shulk an overall "noble hero" appearance akin to an anime protagonist, making this alt fit his personality the best out of all the other alts. Helping is that sick black hair.
Bowser Jr.
Larry (2) has always been my favorite Koopaling. Partially because his yellow skin contrasts well with his blue hair, partially because his clown car's blue accent is complementary to its orange face, and partially because his in-game model doesn't stray too far from his canon appearance unlike Morton (6) or Iggy (5). But it's mostly because his name is freaking Larry. Like seriously, that is a god tier name right there.
Shoutouts to Morton though (6). I like his color scheme, but his in-game model makes him way to small.
Duck Hunt
Two alts stand out to me the most as being possible contenders for the best Duck Hunt alt.
The first is Duck Hunt's second alt. The dog is mostly black with elements of white and brown in there, but what really sells this alt is the duck. I absolutely love the tones of green and purple they chose for it, with the green leaning a bit towards blue. The two colors play off each other nicely and it would definitely make this the best alt if we were to judge the duck alone.
The second is Duck Hunt's eighth alt. The dog still has its brown colors, but they're a bit darker this time, while the duck is this red and black color. The brown color of the dog is tinted red, as is the duck's black color. Combining this with the yellow on the duck's feet and beak makes the color scheme analogous overall.
Both alts are good, but I think Duck Hunt's eighth alt is the better choice for having more consistent colors overall, as the dog and duck kind of harmonize with each other. The dog's eyelids on the second alt is a bit of a turnoff for me as well.
Ryu
All of these alts are kind of similar to me, but the one that stands out to me the most is the black and yellow alt (4). The black is tinted purple, making it complementary to the yellow headband. Plus, the gi also matches with his hair. When you think about the special effects of Ryu's moves in the game, the black gi seems to fit best with them. The gi matches with the black calligraphy of his down special while its dark color also highlights the brightly colored hadoukens. Shoutouts to blue and red Ryu (6) for coming close though.
Ken
I think both of Ken's black alts (2 and 8) deserve a shoutout for similar reasons I chose Ryu's black alt, but the winner for me is Ken's purple outfit (3). The purple is complementary to his hair color, as well as all of the fiery moves Ken has in his arsenal. The gloves even change color to match with his gi.
Cloud
All of Cloud's alts only give slight tints to his usual outfit. I think Advent Cloud looks cooler than regular Cloud. Regular Cloud certainly has better hair, but Advent Cloud has a slicker outfit. It's the cape that really sells it. Out of all of the Advent alts, I like the purple tinted one (8) the best, as the outfit is complementary to his bright, yellow hair. Out of the regular alts, the blue tinted one (3) is the best for similar reasons.
Shoutouts to the 2nd and 6th alts for me literally not being able to tell the difference between them at first.
Corrin
The male Corrin alts actually do a nice job experimenting with the armor color's relationship with the cape color. For example, the red alt (3) makes the cape purple to create an analogous color scheme while the green alt (7) makes the cape red to create a complementary color scheme (7). The female Corrin alts do the opposite, having the armor and cape match but changing the color of the accents. I'm aware that the pink Corrin alt (6) is a pretty popular choice, and I can appreciate its monochromatic color scheme. However, my favorite of the female Corrin alts, and my favorite choice overall, is the black Corrin (8). Score another victory for black and yellow. You have the black armor making the yellow accents stand out more as usual, but the black is also tinted blue for extra contrast. Also helping is that the yellow accents match with the Yato's color.
Bayonetta
Jeanne Bayonetta (5) stands out to me the most for being the only alt to change her hair color, and I appreciate the overall red and white aesthetic. I don't really get the dislike towards this one, I actually genuinely like the color scheme, and looking at it is like looking at red velvet cake. However, I'm actually going to give the victory to the other red Bayonetta alt (6) because, I mean, come on. It's a red and black color scheme. Not only is the color scheme itself cool, but I said before that it reminded me of the Devil, and look at who we're talking about here. It's arguably even more fitting. Adding to that is the Bayonetta 2 design has red attack trails, which fits with the alt's color. Speaking of the attack trails, I'm going to give shoutouts to Green Bayonetta (4) and Yellow Bayonetta (3) for both having outfits contrasting against their attack trails. White Bayonetta (7) also deserves a mention for being monochromatic sans her guns.
Inkling
God, Inkling's alts are all so good. Each one is dedicated to a different color, but unlike the other palette swaps that change same parts of a character in the same way, such as Mewtwo or Wii Fit Trainer's alts, Inklings alts all actually choose clothing to match with the colors they represent. A good example of this is the green alt (4), who gets a green jacket instead of the default Inkling's T-shirt. The jacket also has black sleeves, so he also incorporates black into his design with a black bandana, black shorts, and black shoes (tinted red for some reason), creating a green and black aesthetic overall. It works especially well for the green alt because the weapons always have green accents on them, fitting with his color scheme.
Other Inkling alts that deserve a mention include the popular Purple Schoolgirl Inkling (7). She's wearing a purple skirt, a purple tie, and a black jacket that's tinted purple. However, she's also wearing a yellow blouse that's complementary with the other purple clothing items. The purple also contrasts with the green accents on the weaponry. There's also the pink Inkling (5), who wears black and gray clothing which all have pink accents on them, though the red cap fits pretty loosely with the theme. The nerdy cyan Inkling (6) also deserves attention for being a good representation of an analogous cyan, blue, and green color scheme.
None of these Inklings are my favorite, however. That would be the yellow Inkling (3). It's another yellow and black alt, but this time yellow is the star of the show, and boy does it pull that color scheme off well. She's wearing the hero outfit from Splatoon's story mode, all of which has yellow and black clothing. Yellow is the dominant color for the shirt, while black is the dominant color for the pants and shoes, both of which still have yellow accents. Her headset and shoes, however, has blue lights that are not only pretty to look at, but contrast with her yellow color scheme, as does her eye color (for bonus points, the black on all of her clothing is also tinted blue). If you look closely, you could even see a blue squid symbol hanging from her waist. The yellow is also analogous with the green accents on the weaponry. This alt just does an amazing job playing off of the color yellow, and that's why this is my favorite Inkling alt, as well as one of my favorite alts overall.
Ridley
Ok, I know people will be angry if I don't say it. Meta Ridley (2) is badass. Mecha Ridley (8) is also good, but I feel like it's cooler when something is half robotic as opposed to when something is entirely robotic. Meta Ridley's robotic parts are also tinted purple and red, fitting with Ridley's usual skin color. However, in addition to all of his mechanical incorporations, Meta Ridley also changes the color of his wings to be a yellowy-orange color, which is complementary to the rest of his body as well as fitting with the bright yellow/orange accents on his eyes and chest.
If you want my opinion on what the best Ridley alt is that isn't mechanical, it's the blue one (4). The purple wings are analogous with the blue skin.
Simon
Blue Simon (5) is the best alt in my opinion. The blue armor contrasts with his orangey-brown hair, pants, and skin color. The blue and white aesthetic on his armor is neat to look at overall (probably because it reminds me of his more handsome Echo Fighter), but it also goes particularly well with his side-B, which has the same color combination. Shoutouts to pink Simon (6) and black Simon (8) though for being the runner ups here.
Richter
And now we get to the factually better version of Simon Belmont, who also happens to have better alts. Ones that deserve mentions are his black and yellow alt (7) alongside both of his red and white alts (2 and 8). The green Richter (4) is the best alt, though. It's one of the few alts that change the color of his accents, changing them to yellow in this case. It's another good show of the green and yellow color combination, with the brown of his hair and boots fitting right in. However, the white color on his headband and pants is also tinted blue, leading to a solid analogous color scheme on the alt overall.
King K. Rool
K. Rool's alts are all badass, giving him not only a variety of interesting skin and cape colors that range from being very dark to very bright, but also going as far as changing the tint on his belly. Some of my favorite alts are the blue K Rool with the red cape (3), the pink K Rool with the red cape (5), the black K Rool with the red cape (6), and my absolute favorite K Rool alt overall, the dark red K Rool with the green cape (7). Both colors are dark with low saturation, and they're complementary to each other. The dark colors makes his giant gut and cuffs (both of which are also tinted red by the way) stand out more and look more shiny in comparison. It's also the only K Rool alt that changes the accents on the cape to match with the skin color.
Isabelle
My favorite Isabelle alt has to be the red Isabelle (4), which ditches the plaid pattern on her jacket, makes her ears slightly more saturated, and most importantly, adds both red and blue to the color palette. When you also take into account her yellow fur, it pulls off a good triadic color scheme between the primary colors, with the red being dark yet saturated, the blue being dark and unsaturated, and the yellow being light and generally unsaturated with higher saturation on the ears. The runner-ups for me are the green alt (6) and the pink alt (3), the latter of which pulls off a more subtle triadic color combination.
Incineroar
Black and blue Incineroar (4) deserves a mention here, as not only does the purple tint on the stripes make the color scheme analogous, but the it's all complementary to his bright orange fire attacks. Both of his black alts (3 & 7) are the winner here, and I think it's interesting how they remain with the default's black and red color scheme but apply different tones of both colors in different areas, making the alts look different but keeping the overall feel the same. They're both good, but I think the 7th color is slightly better. You have a monochromatic color scheme with dark tones to match his villainous "heel wrestler" look, but then you throw in the red accents on his chest and hands that really grab the viewer's attention. Bonus points for the black and gray color being tinted blue and purple, giving subtle contrast every time Incineroar uses a fire attack.
Piranha Plant
So all of Piranha Plant's alts are revealed, on the site I provided, and we even have this image to see how he looks in-game. I'm a fan of the green and yellow alt (2) for another approach at a green and yellow color combination, as well as changing the stalk color while still making it look organic. Its purple alt (7) contrasts with the green stalk as well as the yellow pipe, giving it an honorable mention as well. Finally, its blue alt (8) borrows the stalk color from its green alt, not to match with the yellow color, but to instead contrast with it, which I think works out nicely. My favorite Piranha Plant alt, though, is definitely his pink alt (4). The bright, highly saturated pink head (whose white spots are also tinted pink by the way) contrasts with the green stalk, which is dark but still saturated. The blue pot is also dark like the green, but has lower saturation, and the combination of these three colors is just so pleasing to look at. The specific tones they chose for all three main colors just looks hypnotic, making the pink Piranha Plant my color of choice.

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That was a lot to type, but I finally got through every character. If you agree/disagree with me feel free to comment for further discussion. Feel free to use the template I provided on my Google Sheet to share your own choices as well. I'll keep this updated for Joker + the future DLC fighters when they're announced.