I've become so used to the movies that I can't remember (nor find) if there is a passage in the books that explicitly states if the Hogwarts uniforms (that are not as depicted in the movies, I know) reflect one's House membership and, if so, when they start doing so. When bought, the robes must be plain black, of course:
HOGWARTS SCHOOL of WITCHCRAFT and WIZARDRY
First-year students will require:
- Three sets of plain work robes (black)
(Harry Potter and the Philosopher's Stone, page 74)
But what about after one has been sorted? Do they stay black regardless of the House?
I'm not asking about the movies and what kind of magic would be used - as in How do Wizards get their house colours? - but how the uniforms are depicted in the books.
I've found further details that substantiate the accepted answer:
The first time Harry takes the Hogwarts train Percy - Gryffindor in his 5th year - is said to have already changed into his robes and there's no mention of colors or House badges - only his prefect badge (emphasis added):
“Where’s Percy?” said their mother.
“He’s coming now.”
The oldest boy came striding into sight. He had already changed into his billowing black Hogwarts robes, and Harry noticed a shiny red and gold badge on his chest with the letter P on it.
(Harry Potter and the Philosopher's Stone, pages 106-107)
When searching for the keywords #robe and #badge I found only prefect and Quidditch captain badges (aside from "Harry stinks", Cedric Diggory or Weasley is our king badges) and colored Quidditch robes.
Also, when the Gryffindor Quidditch team is training for the first time in CS7, Wood is not able to distinguish Colin Creevey as a Gryffindor student.
In CS12, after having drugged Crabbe and Goyle, Harry and Ron steal only the shoes from them, while they use "spare robes" stolen by Hermione "out of the laundry". In both cases, it's because of the size difference, not because of House-identifying colors or symbols.
Students occasionally wear scarfs in the colors of their House - mostly during Quidditch matches. This would explain how Harry is sometimes able to tell to which House someone belongs, even without knowing her/him.