GameTooltip_UnitColor(unit) is the function used for setting the reaction color in the tooltip. You could probably hook that without taint problems.
In WoW 3.0, the UnitReactionColor table has been replaced with the function:
Code:
ref, green, blue, alpha = UnitSelectionColor(unit)
Which is called in a bunch of places. I've no idea if hooking that would cause you taint issues.
-- Cirk