I'm writing about a fictional human ethnicity where everybody have black hair. I like to describe how my characters look, but the problem is that they may look very similar when it comes to facial shape, nose shape and (especially) hair colour. It is a very homogenous group. When doing multiple descriptions of the looks of characters, is it necessary to describe individually that they all have black hair when it is a well-known fact that "all people in that country have black hair"? Is it enough to describe that "all people in that country have black hair" once for the readers to understand it?