Hold down either left or right click to paint. to change colors you can press "B" key for blue, "R" key for red, "Y" key for yellow, "G" key for green, "O" key for orange, "P" key for purple, "P + 2" keys for pink, "B + 2" keys for brown, "W" key for white, "G + 2" keys for gray, "B + 3" for black. If you press the "D" key it will erase everything on the screen. The "1" key is the smallest size while "0" key is the biggest size for the brush.
I got this idea from CooksterKid if you want to see his projects: @CooksterKid. This honestly didn't take me long to make so if you see any bugs please tell me so that I can fix them!