True or not, she had better come up with a better story than that for the people who come looking. If I went to look at a potential purchase and the seller told me that they really loved the horse, but just couldn't deal with her color, I'd feel 100% certain that there was something major wrong with that horse that they just weren't disclosing.
I bought a cremello horse a few months ago, and he doesn't actually get all that dirty. Maybe if they show a lot at rated shows, having a white horse might be enough extra work to want a darker color. Maybe.