When I got the case for my iPhone it was a lovely turquoise. In the past year and a half it has faded, or something, and is now a dull, slightly bluish yellowy green, sort of. I was thinking of buying a new case, but then I thought "I could tangle it!". This took part of an afternoon and I'm very happy with it!
I also tangled the front, top and bottom. However, it's a different sort of plastic and the marks were not as crisp and it started rubbing off after a few days. I removed the plastic covering and now these two parts are parts of history.
I used a Sakura Microperm pen and I'm becoming quite a fan. It can write on all sorts of surfaces and is what we used to tangle clam shells at Retreat to Paradise in early April.