The color scheme you refer to is probably most famous due to it being what's used for World of Warcraft, where, in order of increasing rarity:
grey < white < green < blue < purple < orange < red/beige (developer items only).
I'm not sure that that schema is that common though -- Blizzard's own diablo II used grey < white < blue < yellow < purple (I think), and games like Torchlight (grey < white < green < blue < orange < purple) vary slightly.
Actually, both of those are slightly flawed comparisons, because in diablo II, green items were used as "set" items, beige were "unique" (only a single copy could drop per game). Bottom line -- I think the trend is more varied than you think.