How to color the back of your iPhone?

I dropped my WHITE iPhone 4 two weeks ago and I came across a picture of one where the back was cracked (as mine is) and it was rainbow colors. I would like to try and do this to mine, but I don't know how.

It would be great if you could tell me.

Simple, get a bunch of marker pens and draw over the cracks. (Not sarcasm, this is actually how you do it)

Buy a case