I hate mustard, so why would I want it adorning my beautiful iMac?
Come to think of it, I'm not the biggest fan of yellow pee, either.
Lastly - those photos? Horrible Photoshopping there. I could do a better job with my toes.
I seem to remember a company that sold face plates for the iMac line back during the Rev. B days (Summer '05). In fact, I remember some spam from that site. I hated it because it covered up the Apple logo.
Alas, if Apple had intended for these items to be colorful - they would have given us options in the first place. I'll stick with white, thank you very much.