They would have to change color scarves. The robes are black though either with the Hogwarts crest or they sew on their house crest. Whoever said that they didn't mention scarves and ties in the book is wrong.
And James totally wearing those weird colored scarves! Lily and James would be so sweet at parenting! but the Weasley's were not in the order the first time around. The book of The Order Of The Phenix said so.