What else is there to do with a cracked ping pong ball?
Sorry for the lack of photos, but I think it should be easy enough to figure out without them =]
You should see a ridge along the center of the ping pong ball. Paint one half of it red, and the other half white. It's okay if the line isn't straight between them for now, because you'll be painting over it in black.
If you're using an obnoxiously bright orange ping pong ball like I was, you may need 2 or 3 coats of paint for this step.
Take a piece of masking tape and wrap it around the entire circumference of the ball about 1 cm above the center line. Do the same with another piece below the center line. Now you should be able to paint on your black line perfectly.
You're going to have to be really careful with this next step:
Draw on a black circle with a fine brush. I sketched it on with a pencil first to get an idea of where I wanted it.
MAKE SURE THE BLACK PAINT IS DRY.
Then fill in the inside of the black circle with white. Again, you may need a few coats of this.
You can also use this time to go back and neaten up any lines necessary.
Wait till it's all dry, and tadaa! You can now train to be the world's greatest pokemon master ;D