In my case when I bought the Happs trackball that had the bulb preinstalled with the 3" blue translucent ball, and like a lot of other people I've heard about, it was enough to heat the ball for an hour and make it expand so it wouldn't actually spin right. So my solution, wire up a bright blue LED to a 12v adapter, with the correct resistors. So now all I gotta do is plug in my LED and put it underneath the trackball, it's extremely bright blue shining on translucent blue, results a VERY cool looking milky-ish blue.
-CthulhuLuke