That's not a white balance problem that is an exposure issue, white balance is for colour cast like the yellow of incandescent bulbs ... the meter on your camera meters for middle grey (12 or 18% depending on who you ask and which manufacturer of camera/light meter) ie it tries to make the metering area (this depends on the mode your camera is in) look middle grey. If you take a photo that is of a mainly white object the camera will underexpose and the white comes out grey.
If you have a manual mode on the camera either select a shutter speed upto half what the meter says or select an aperture upto 1 stop slower the picture will probably be better exposed. If you use Aperture priority, Shutter Priority or full auto (or SV mode on the Pentax K10/20D) then you can dial in exposure compensation which will have the same effect.
Also If you are using an DSLR (or any camera that can use it) then use RAW - you wont have any white balance problems then.
Barney