-Rendering is basically how a game engine applies various effects to something.

-All of the lights you see in a map all have to be calculated out in order to create shadows, and shade everything according to the properties of the light.

-Since half life is a 5 (maybe 6 now?) year old game, it isn't very complex.

-A great thing about changing renders is that no matter what combination's you enter, you can't crash the server. (Unless its a crappy unstable server)

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First, and most important is the Mode (render mode)
Words in bold are what you enter after the command

0=Normal (brush entities, models, sprites)
1=Color (brush entities, sprites)
2=Texture (brush entities)
3=Glow (sprites)
4=Solid (brush/solid entities)
5=Additive (brush entities, models, sprites)

-The mode will affect what you're re-rendering the most, it changes what the rest of the commands (after mode) will change.

WARNING: Using mode 3 on something other than a sprite, will cause a very annoying console spam saying something like "none sprite set to glow".
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Next is the amt/amount.

-This should have really been placed after the effects and r g b variables, since they are closely related.

-In most situations (depending on what mode and FX you use) this will edit the transparency.

-For this parameter, you enter a number 1-255. 255 will apply an effect the most (the most opaque in most cases). 1 will make the object completely invisible (in most situations).
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Now we get to FX/effects

-These will stay the same no matter what mode you are in, but may not be visible.
-Depending on the amount (last section) the effect will be the most visible.

-They work best in modes 0,1,4,5

Possibilities:
1-19
Since the descriptions valve gives them aren't the best, I suggest you play around with all of them, and remember ones you like. Here are my favorite ones:

0-nothing
1-Slow Smooth Fade (in and out)
2-Fast Smooth Fade (in and out)
8-Slow Flicker (random in and out)
9-Fast Flicker (random in and out)
10-Slow Consistent Flashing (off and on)
11-Fast Consistent Flashing (off and on) Will hurt your eyes
15-Liquid Fast Flicker (very quick fading in and out)
16-Hologram (models and sprites only) It will look like 15, but will jump every once in awhile. The further you get away from something with this effect applied, it will become completely invisible.
19-Glow Shell (models only) This is the most famous render effect. It can be seen in Quake 2 and 3. This is the colored mesh looking thing that surrounds a model. Its the effect used with Invunerability and Quad Damage, and AdminMod's "glow red" "glow white" "glow police" commands. You can NOT have a black glow shell.
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R G B
Red Green Blue

-Ever look really close to your TV? Each pixel is made up of red, green, and blue bars. Different intensities of these colors mixed together will get you any color your eye can see.

-A more scientific coloring technique is C M Y L. Cyan Magenta Yellow Black. This is what printers use in order to get a much more detailed picture. You don't need to worry about this...

you will enter different intensities of these colors using numbers 0-255. You mix them together to get different colors. Its kind of like mixing paint.
Numbers to keep in mind: 255, 128, 64, 0

255 0 0 - red
0 255 0 - green
0 0 255 - blue
255 255 255 - white
0 0 0 - black
255 128 0 - orange
255 255 0 - yellow
128 0 128 - violet
255 0 255 - magenta
0 255 255 - cyan

Try different variations to get some cool looking combinations.
128 128 255 - electric blue
0 128 64 - army green
128 64 0 - maroon
128 255 0 - neon green
255 128 255 - neon pink
128 128 128 - gray...
255 255 128 - neon yellow
128 0 255 - sky blue
0 128 128 - teal

I figured most of these out on my own, but a good aid in figuring out colors is using MSPaint. Double click on a color at the bottom of the screen, click "Define Custom Colors", move the curser around the color gradient and look at the red green blue values.
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So, here's just some render commands you could try to get you started...
You could use admin_render persons_name_here before the other commands in order to apply a render on somebody who currently isn't near you, or for targeting yourself.

On a model:
e_render 0 1 19 255 0 0 - red glow shell
e_render 5 1 19 255 0 0 - red crystal glow shell
e_render 1 1 19 255 0 0 - red cloak glow shell
e_render 0 255 19 255 0 0 - red messy glow shell
e_render - shiny!
e_render 5 255 16 0 0 0 - hologram
e_render 1 0 0 0 0 - invisible
e_render 0 0 0 0 0 - reset (completely opaque with no effects applied)

On a solid brush:
e_render 1 255 0 255 0 0 - solid red
e_render 1 128 0 255 0 0 - transparent red
e_render 1 255 0 0 0 - solid black
e_render 1 255 0 255 255 255 - solid white
e_render 4 255 0 0 0 0 - hides the decals (blood, bullet holes, sprays)
e_render 2 128 0 0 0 0 - textured with transparency (glass)