White appears to be a neutral color. Wild magic is white unless influenced by an outside power. Caerroil Wildwood's eyes glow silver. Some of the Elohim glow with white light. Sandgorgons are white.
Blue is the color of the Creator's eyes. The Lords wear blue and Lord's-fire is blue. The Forbidding set by Kevin to guard the Second Ward is blue.
Green as grass green is the color of the forests and growing things and the power wielded by the Colossus of the Fall. The lost One Forest probably wielded green power. The emerald green Illearth Stone represents the antithesis of the growing things; it kills all vegetation near Stonemight Woodhelven, corrupts its wielders into blood-thirst and nature-hate, and is readily used for warping living beings. Dead Elena appears dressed in emerald green with an aura of emeralds. Kevin's eyes drip green when he returns to fight Elena. Dukkha's skin has a green shade instead of the normal Waynhim gray. The graveler of Stonemight Woodhelven wears emerald green. Esmer's emerald eyes present a significant hidden threat about his future role.
Yellow is the color connected most solidly to Earthpower. Hurtloam gleams golden. Lord Mhoram's eyes have golden spots. The activated Damelon's Door is yellow. When Linden wields the Staff of Law, the effect is yellow. The tainted Earthpower of Kevin's Dirt is a dirtier shade of yellow. Lord Foul's eyes are carious yellow.
Orange is the color of flame and on the Earthpower continuum from yellow to red. The Wraiths of Andelain look like flames. The Mahdoubt's other eye is flame-orange.
Red is the color of EarthBlood and the deep end of fiery magic of the stone, but more prominently the color of various evil magics. Ur-vile blades glow red. Drool's corrupted moon glows red. Cavewight eyes are red, as are Gibbon na-Mhoram's. The Clave wears red and black, and the ray that connects them to the Sunbane is red. The secret doors in Ridjeck Thome and the Clave-controlled Revelstone are limned in red when activated. The rubies scattered among the diamonds of Infelice's attire hint about her corruption.
Spoiler
Violet is a color of mystery. The Mahdoubt's other eye is violet, and the question is, does it belong to the extreme blue or the extreme red end of the visible spectrum? Violet may be associated with mind control, but we don't really know...
Black is the color of Demondim magic. Both ur-viles and the Waynhim use black magic and ur-viles are also black themselves. Perhaps Demondim-magic is actually infra-red which simply appears to humans as black. Lord Foul's venom is black.
Multicolored is what caesures are. The Elohim as a group are also multicolored. When white light is broken with a prism the effect is a multicolored rainbow. Caesures represent a local breakdown, and I think it can be argued that the Elohim are pieces of the sleeping Würd, which would presumably be white.
By the way, I'm noticing that the system in the Chronicles might be connected to the color system detailed in The Hog by William Hope Hodgson. (Check out the link for the freely available classic horror story. It's good and scary.) I'm really wondering about the same author's Night Land (called simply the Land by the inhabitants) too.