Dumb question inc; Do white boards/caps start to turn crusty colored after awhile?
The "crusty" color you're talking about, which everyone has seen thanks to the older "white" computer cases and CRT monitor cases, is actually caused by a two different factors. The plastic used on those white computer cases is actually ABS (same as these keyboard). However, ABS is flammable, and computer/monitor cases generate lots of heat and have high voltage electricity running through them, so to meet safety standards Bromine is added as a fire retardant to the ABS. Bromine reacts chemically to heat and UV light, and this chemical reaction causes the ABS to start to turn brown, which combined with the white pigment makes the yellow or "crusty" color. Since most computer parts are not exposed to sunlight, it's mainly the heat that causes the discoloration over time.
Keyboards do not generate heat (your fingertips are probably warmer than any heat generated in the keyboard itself). My last white keyboard was 10 years old and had not started to discolor at all. Not to say they wouldn't do so under certain circumstances, such as being left by a window where the sun shines on it every day, but they will discolor much less than computer cases and monitor cases.