In all likelyhood, the white vehicle idea is a ringer.IMHO.
Again and again, I try it...I look into a parking lot. The outstanding vehicle is white. It almost glows. The next color I see is red. The rest of the vehicles are dark and relatively indistinguishable.
Whether it is "tunnel vision" created by the report of a white van/truck or the norm, I don't know.
Maybe there is a dummy "clean" van at each site or....it is just the "odds" of a white van.
Sac