As finfan said - for some reson they put a few coloured LEDs under the 8100's trackball but didn't on any other BlackBerry; including the 8110, 8120 and 8130. Weird, huh?
the best i've found is using a post-it note or just regular paper and colouring it with a marker or hi-lighter and sticking it under the ball where the light it, that way, you can just take it out for warranty. and... the ball still works like normal if you use the right thickness of paper.