Was also surprised when I first washed the white car after winter. I was like wtf is the new car rusting but soon found out it was flying rust stuck on the paintwork. It all came off when I used iron removal. Of course that's been the same with my all previous cars too, but as they were greyish the spots didn't show like they did on white car. I thought that would be more of a problem in here, since most Finns use spiked winter tyres (which I hate) and I thought it's the spikes that cause the flying rust.
White and black cars are indeed painful to maintain clean. After winter you need double wash to really get all the dirt off. Grey was so much more forgiving, though funny to know afterwards how dirty they still were when they looked quite clean.