Yeah, I got the white Belkin bumper case along with my 5S as a gift last week. The outer, soft rubber parts of the bumper have turned into an ugly yellow colour which has been a ***** to get out, to no avail. But to my understanding, this is the case for any other bumper case out there.
Nonetheless, at $30, save your money and don't buy it.
Family members have the Spigen Neo Hybrid EX Slim bumper ( with the white silicone and various polycarbonate rims) that has held up well for 2 months now. No color change. It seems coated with gloss finish that is less affected by UV or for absorbing color.
I've had the black silicone version for months. Works well
PS I just saw a friends 6 month Spigen bumper in white. Held up well. No color change.