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.
Person: EVERYBODY CLAP YOUR HANDS Mugglborns/halfbloods: *Claps violently* <<<<<<some kid gets is trouble and the teacher just goes,"Why?" And then all the muggleborns are like,"MCA, its to stay at the YyyyyyyyyyMCA!