@SoleneMa & @bengold - I saw a few examples of code to do this (and have used them myself) whilst developing ColourColor. I'll dig them out and get back to you.
EDIT: here's a solution by the guys at Panic. It's for Mac (based on the coloured cells in iTunes 11), but can easily be converted to iOS. If I find the other examples then I'll let you know.
http://www.panic.com/blog/itunes...
This is awesome! Reminds me of what Pinterest does as thumbs are loading:
Pinterest Makes Loading Smoother By Displaying Primary Image Color Behind Images: http://productdesigninspiration....
ReqRes
Adaptive Backgrounds
Call To Action
Lydia
Moleskine Studio
Dododex
1Degree
Adaptive Backgrounds
SpacedTime