It's pretty fun if you have other colors to use. I just used white for now because it was easier than figuring out how to make colors quickly interchangeable.
What you do is set up your tripod and point your camera at your subject. Then set the shutter for as long as it will go (the longer the shutter, the more time you have to "paint") then you can adjust the aperture so that it doesn't get over exposed.
Then, take a bright, powerful light (I used a spotlight), trip the shutter first of course, and shine the light on your subject anywhere that you want lit up.
It's a pretty neat way to play with light and shadows.
Here's a golf ball I was playing around with one day:
I used white, Red and Green light:
So have fun with your "painting"!