Well, after 3 years, my trackball looks less like a pearl and more like a turd. They yellow into a not-so-nice looking ball after a while.
Although, I use Blingball to light it up and it takes the yellow look out when it does light up. I have it set to "white" which turns all the LEDs on and gives it more of an "opal" look with the various colors being more prominant depending on which way you look at it.
I know I can get a replacement ball cheap, but I just never got around to it.