“is a college’s racial/ethnic composition an unstated overriding factor for many people,”
Yes. It is a dealbreaker for the vast majority of white people. They do not consider universities where they will not be in the majority. This preference is invisible because whites are in the majority at the vast majority of U.S. universities.
For people who aren’t white, there’s a lot more variation. Some seek schools where they will be in the majority; most look for schools where they won’t feel isolated; a few don’t care.