Well, yes and no. There's two major components to visibility, the light intensity and contrast with the background. Against a background of fresh white snow, a white car will obviously not be the best choice. However, in most cases winter driving, at least where I live, is not driving in fresh white snow. But rather snow around the roads is kind of dirty and grayish, frequently the snow has been eroded away from the road surfaces. The problem from a visibility standpoint is that it's often kinda dark as the sun doesn't get that high in the sky in the winter and often during normal commute times the sun is down. In such cases a white car has an advantage, being the most reflective color.
So all in all, well, I haven't seen a study wrt winter visibility, so I'm just making things up, but I'd think white is far from the worst, certainly better than dark colors. Probably some color like lime yellow or such that you frequently see on emergency vehicles would be better, but few people choose a color like that for a civilian car.