You can paint on Fondant or a layer of royal icing, or wafer paper--though fondant is the easiest way to get a good result. Regular icing gel color is mixed with a small amount alcohol--vodka, gin, everclear, or lemon extract (83% alcohol)--which allows it to be transparent, like watercolor, and dry very fast so your fondant doesn't get gooey. A little water doesn't hurt either. If you want a more opaque color, or a softer shade, adding a little "White White Icing Color" by Wilton will do the trick. Luster dusts, or petal dusts can also be made into "paint" by adding alcohol. I usually use a clean soda bottle cap as my little paint pot. Large amounts will dry too fast.