There is a trooper in my garrison that has multicolored armor parts. It bugs the hell out of him, but his looks best on film and in pictures. We've got a local guy doing a fan film who tells me that all the others look like a white glare on film. Apperently that's why Vaders helmet was painted checkerboard black and silver grey.
If you do decide to paint, I recomend Krylon Fusion. They offer a few different shades of white. But I caution spraying over old paint because you never know what sort of efect you'll get ie: orange peel and crackle.