When I start in on an interface inventory one of the first things I almost always notice is an over use of colors. Piles of different colors of slight variation throughout an app. Of course a bunch of different, slightly similar colors look disjointed, or, at worst, causes confusion about what is actually clickable/tappable. But I always get to wondering about the grays (assuming they're not used for interact-able elements). It can be terribly useful to have a number of grays to subtly layer elements in the visual hierarchy of an interface.
Do you limit yourself in the number of grays you use? If so, why and how many? (something something 50 shades)