It makes more sense if you've read tOotP, but all throughout the books, you hear stuff about the Weasley sweaters. Ron has a maroon sweater, as we hear him so often complain. Charlie might have a red sweater. (Because of the dragons, of course.) Bill might have gray, or a nice sage colour. The twins could have a light blue type thing.
Anyway, I find that what a person wears tells something about them. So, I figured it was important.
