Re: D & D
Found it: The Initiate of the Sevenfold Veil (pg. 44 Complete Arcane)
"Colors are imbued with their own magical properties. Just as exotic substances and mystic words have the power to repel certain creatures, colors also harbor innate properties that can be brought forth by the skilled mage, culminating in the perfection of the prismatic wall or prismatic sphere...
A master of defensive magic, the Initiate of the Sevenfold Veil approaches its prismatic barrier by mastering by mastering one of its constituent veils or layers. Her skillful wardings can prevent many of the most perilous attacks enemy spellcasters or monsters can muster, shielding herself and her companions against harm."
It's essentially a defensive color mage, if I'm not mistaken. The ISV creates a "warding" and and can imbue a color into depending on its level. It starts with red and as its levels progress it learns orange, then yellow, then green, blue, indigo, and violet. Each type has a certain type of defense ( red=ranged attacks, orange=ranged magic, yellow=gases/poison, etc.) and some colors gives buffs to players inside the veil.
If you've ever read the Seventh Tower series, it's something along that line.