Its a bit expensive for an interior light (£37)BUT it will cycle through 2 million colours and can be linked up tolots of other stuff if you want to expand the system.
It starts off white and then after a few seconds it then begins to slowly cycle through all the colours. If you momentarily switch it off and back on, then it will lock to whatever colour it is showing at the time.
They also do a 'rough' version that can be fitted to the exterior. This is even submersible. They come from Iceled who make them but are also available from litewave (check out the gallery there and also the Electrik decal page and see if you can recognise anyones car on there

oops: )
www.litewave.co.uk