Well, I must be a bit older than the other respondents but I certainly remember white Bell helmets being very common in my younger days. I never paid attention to what the Harley guys wore versus what anyone else wore, so I can't confirm or refute the idea that Harley riders used to wear them. But if you were to dig into the Bell archives and inquire about color choices back in the 50's you might find that white was very heavily represented. I've always been partial to either black or red btw, but even in the late 60's when I bought my first skid lid there were plenty of white helmets on the display racks.
2k, as someone who has owned many black cars over the years, I have to disagree with your statement that white shows dirt and scratches more than black. Black shows every tiny bit of dust and every smudge or swirl, not to mention scratches. On the other hand, white or other light colors tend to hide dust and light scratches, at least until you get up close. People who don't want to wash their cars often (and by hand) tend to go for white and light tan, definitely not black.
Jerry