Add a new type of expressions menu control that allows a user to select RGB or RGBA values. The values would update 3 separate floats (0-1).

It is possible for an avatar creator to create this functionality in the current system, but the end result is not a very user-friendly interface for in-game users. The in-game color picker could be a color circle with darkest values at the edges, white at the center, and full-saturation in-between.