I tried a few shots against a white wall the other day using a 580EXII and an AB800. The white balance on the camera was set to 7000K, but all the pictures had a blue cast. I tried setting a custom white balance based on the overexposed white wall by itself, but that made the pictures extremely magenta. I was shooting in RAW, so the blue cast was an easy fix, but is there a way to fix this in the first place? I suspect the answer involves flash gels, but I wanted the advice of someone experienced before I spend the money.