Disassemble your board and lay the deck on something like newspaper that can get dirty. It should be outside or in a well ventilated area.

Paint it with either a single color or some sort of design and let it dry. If you want,you can sand it before applying the paint.

Pour dish soap over it in any way you like,which will "protect" the color underneath, spraypaint the deck with another color and let it dry.

Wash off the soap thoroughly and dry with a towel,but don't do this too harsh as it could rub away some color.
Cover it with a clear coat paint and let it dry.