Actually I haven't watched too much anime and Re:Zero is not seen. But I like figures and in one of the anime sites I do what to add figures to the database and edit information about them. Because of this I have remembered how to look like many of the characters and where they are.
And in this case, the hair color really plays an important role, since REM is rozovolosaya twin sister RAM. Here is a figure from Souyokusha, it is clearly seen that girls differ only color of eyes and hair: