aperture and shutter speed help

i am set up for some wedding portraits. i have flooded my white paper background with plenty of light and my subject is approx. 6 feet in front of the background. i am shooting with a ST-E2 remote to two (2) 420EXs. the main flash is 36" from the subject with a LumiQuest mini softbox and the fill flash is reflected from a 30" umbrella (also at a distance of 36" from the subject). the goal is high-key lighting and both flashes are at 45 degrees on the left and right, respectively.

when i shoot f11, 1/125, i get a great shot of the subject but the background is slightly grayish. at f13, 1/90 the background has a pinkish tinge. any recommendations to get a pure white background? thanks.
-sbk

Aren't colour shifts a pain?

More light on the background.
Play about with white balance in camera.
Play about with white balance on PC.
Filter - to cool the pink, but watch the skin tones if you go too far.

Best of luck