A small, clear, drop of water on the shiny surface of a disc within a narrow beam of light. I also used a handheld colour-changing LED above the DVD to give extra colourful effects. The colours aren't enhanced or altered in any way.
I haven't done this for a while - I may try a few more sometime soon as I've run out of time for today!
Sony Alpha 100 SDLR camera with Cosina Macro 100mm lens: 1/40 sec at f/18 (ISO 100)