Select the color you want to start with. Use the number keys to change pen color. The slider in the top left corner changes pen size. H hides the pen size selector, S shows it again. 1=red, 2=orange, 3= yellow, 4=green, 5=blue, 6=indigo, 7=violet, 8=magenta, 9=black, 0=eraser, space=clear all.
I think I was probably inspired by Mario Paint for the SNES. I had my game make a popping sound every time it stamps because it reminded me of Mario Paint, which did the same. THE FASTER YOU MOVE THE MOUSE, THE FARTHER APART THE STAMPS ARE PLACED!!! if you want a smooth line, move it slowly.