Hmm. I've wondered this for a while. In the movies, the students have different coloured ties and logos on their robes. Is there any evidence in canon of that or is it just a movie thing?
I don't know but I would assume it was a movie thing as how is a first year buying their robes for the first time meant to know what house they are in, that always confused me. But I suppose there would most likely be spells that could put the colour on the robes. I just always thought they were plain black.
Cubicle Conversations with Myrtle
In canon, students are asked to buy black robes, a black cloak and a pointed black hat. Anything else is illustration/movie canon. Logically speaking, if you look at canon, wizards don't wear anything under their robes (except of course underwear - although presumably dotty old Archie doesn't).
OK. I think I'm going to stick to your explanation, Minna. Ta, and ta to Sarah too!