Decently Optimized Colours

Water + Wind -> Wave
Earth + Earth -> Mountain
Earth + Wave -> Sand
Fire + Sand -> Glass
Mountain + Glass -> Crystal
Earth + Crystal -> Gem
Fire + Gem -> Ruby
Mountain + Ruby -> Red

Water + Fire -> Steam
Water + Earth -> Plant
Fire + Steam -> Engine
Wind + Plant -> Dandelion
Plant + Engine -> Car
Engine + Car -> Racecar
Dandelion + Racecar -> Yellow

Red + Yellow -> Orange

Fire + Earth -> Lava
Water + Lava -> Stone
Lava + Stone -> Obsidian
Earth + Obsidian -> Diamond
Plant + Diamond -> Emerald
Fire + Emerald -> Green

Water + Water -> Lake
Fire + Wind -> Smoke
Water + Smoke -> Fog
Plant + Fog -> Mushroom
Mushroom + Mushroom -> Giant Mushroom
Fog + Giant Mushroom -> Smurf
Lake + Smurf -> Blue
Fire + Blue -> Purple
Blue + Purple -> Indigo
Red + Indigo -> Violet

----------------------------------------------- VISIBLE COLOURS ARE DONE -----------------------------------------------

For all the light waves, because why not at this point

Wind + Wave -> Storm
Fire + Glass -> Lens
Storm + Glass -> Lightning
Water + Lens -> Microscope
Lightning + Microscope -> X-Ray
Lightning + X-Ray -> Gamma Ray

Engine + Engine -> Rocket
Rocket + Rocket -> Satellite
Wave + Satellite -> Radio
Fire + Radio -> Microwave
Microscope + Radio -> Radio Wave

----------------------------------------------- THE WORDS COLOR/COLOUR BELOW -----------------------------------------------
From the start to the word Color or Colour (Colour is 3 recipes harder...)

Water + Earth -> Plant
Earth + Plant -> Tree
Fire + Tree -> Ash
Tree + Ash -> Pencil
Ash + Pencil -> Crayon
Water + Crayon -> Color

Water + Earth -> Plant
Earth + Earth -> Mountain
Earth + Plant -> Tree
Wind + Mountain -> Avalanche
Fire + Tree -> Ash
Earth + Avalanche -> Landslide
Tree + Ash -> Pencil
Mountain + Pencil -> Crayon
Landslide + Crayon -> Colour