The color difference helps you tell what emotion or physical state that Mudokon is feeling or in.
Blind ones are gray and colorless, most likely because their world is colorless and dark and that translates onto them.
Mad ones are red with anger
Sad ones have the "blues"
Hyped-up Muds are probably yellow because it's a bright, vibrant color and so are they
Drunk Muds sick with brew are green because green is the universal color for a sickly feeling.
Abe's color is explained in the book. He's from a more arboreal climate, so he has a mottled purple-green pigment.
Other muds are green, and I suppose that's to tell them apart from Abe.
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