Mac OS X color profile bug
I've been experiencing this bug every once in a while with my MacBook (running Tiger). Basically, when you use fast user switching, there's a chance that the color profile changes to "Generic RGB", which makes colors look bright and washed-out. (Changing the color profile back in the usual way has no effect.) This puzzled me greatly at first, but now I know that I'm not crazy. :-)
While a reboot resets the original color scheme, there's an easier way: run the DMProxy script and everything returns back to normal. (More details in the link above.)