Perhaps the bakers around here will chime in with more specific information, but I believe that the difference between red and white wheat has to do with protein content, not actually color contribution to the finished beer. Red wheat is higher in gluten and tends to get used more for bread, whereas white wheat is lower in gluten proteins and tends to get used more for pastries.
I believe most brewers use white wheat most of the time, but I've heard of pro-brewers switching between the two in a pinch without much noticeable difference. If I were to speculate, I'd think that a bit more protein would give a hefe a style-appropriate head-stiffness, so perhaps that is the reasoning.