I have had my blackberry curve from VZW for approximately 10 months. Recently I have noticed the track ball is not longer the off white color it previously was but now is darker in color and it appears to be reddish in color now, kind of like the light went out or something...
It is probably just dirty. You can try to clean it but good luck. You will get it a bit whiter but never close to the original white. You can replace it with one from ebay for 4.84 and no shipping charge if you wish (seller mobilexny), as well as places like cnn.cn or bbenriched.com for around 7-8 bucks.
Cnn.cn has them in many colors as well if that suits you.
The trackball is just getting dirty from the dirt/oils on your hands. There isn't really much you can do, as it is absorbed into the ball's surface because the plastic is textured and slightly porous.
If it bothers you, you can buy a replacement trackball assembly and change it out for the new one.
