Single color picker
Color Picker is based on HSV color model. Using color wheel in the middle, user can adjust hue and saturation. Arc on the right side allows user to change value of the selected color. Arc on the left side shows currently selected color. This is how it looks like:
Multi color picker
More advanced version allows to pick several colors at once. It is really not easy to compose a palette of eye-catching colors. Changing only the hue while keeping saturation and color value on the same level gives nice effects and is easy to achieve.
Project is available at GitHub: https://github.com/chiralcode/Android-Color-Picker/
Superdry Color Picker