I was thinking about the color wheel and then I though, a color sphere could be interesting.
A spherical color wheel is actually fairly baked, but an input device for artistic types might be useful.
The idea is a track ball (tracked by a fancy laser system, so there are no alignment issues) with
colors printed on the sphere itself.
On the "equator", the colors cycle around like on the usual color wheel. One pole starts from white and along each longitude line the colors go from white through their particular color to black.
A small touch screen on the side of the ball acts as a slider for saturation.