This is the part that I was thinking about because I think most look better with their natural hair. On the other hand, it would be too much work to police who is natural or not. Even some of the most famous 'blonde bombshells' were not natural blondes (as an example). The easiest thing is to just classify them as whatever they had for the majority of their success. On the other hand, most likely exclude anyone who changes every other week. There are plenty of models who stayed one way or the other for long enough to make both sides competitive.