I think the thing is that since the colors of the Divine 9 are more well known, ( I doubt many people could name the colors of all NPC sororities) and don't repeat colors of the other organizations that it is a little more like "stepping on toes" to wear colors of a different sorority/fraternity in NPHC.
In Phi Sig when we are doing things for the whole chapter they are generally in Phi Sig Colors (King Blue and Gold) but if it is something that JUST you are going to wear.. then it doesn't really matter what color it is. Most of the things that I have in non "phi sig" colors is patterned anyway.. I don't think ANY organization has delclared that purple leopard is their color... so I'm pretty safe wearing my purple leopard letters on a black t shirt.
