2 circles. Both: 256x256. 1 hard edge, 1 smooth edge. Both have alpha transparency.

This is used with HSV or HSL. Whatever you want to call it. I would say HSL (Hue, Saturation, Light). This circle displays the 2 axes Hue and Light. Saturation goes deep down into 3rd dimention (e.g.: red, dark red, draker red, black). Well, I developed my own HSL algorithm. The result is correct so I'm very satisfied.