An exploration of dress robe colors and houses, as found in book four, Harry Potter and the Goblet of Fire.
During the Yule Ball scene of Goblet of Fire we are given descriptions of several dress robes, thirteen to be exact (fourteen if you want to count Crabbe and Goyle as separate entities). The list of characters can be divided into two main sections, students and non-students. These two categories are further divided into those who are wearing their House colors (or old House colors), and those who are not. The colors of the robes reveal something about the character wearing them, more specifically they show a different House the character could have been sorted into, if the character is wearing one of the House colors.
Here is a list of the characters and their robe colors: Ron, maroon; Harry, green; Parvarti, pink; Padma, turquoise; Fleur, silver-grey; Draco, black; Pansy, pink; Crabbe/Goyle, green; Hermione, light blue; McGonagall, red tartan; Bagman, purple with yellow stars; Madame Maxine, lavender; and lastly Percy, navy blue.
To start there are the non-student characters: Fleur (she isn’t a Hogwarts student and thus isn’t sorted into a House), McGonagall, Bagman, Madame Maxine, and Percy. This is the only group where some of the characters in it are not wearing one of the house colors (gold, red, green, silver, blue, bronze, black, and yellow), and I will start here. Madame Maxine is wearing lavender robes, as she never went to Hogwarts this seems to be an indication that she is in no way affiliated with any of the Houses.
The other adult characters are wearing House colors. Fleur is wearing silver-grey robes, which makes for some interesting suppositions. Though she is not in any of the Houses, silver a color of Slytherin House. Too little is currently known about her character to say if she could belong in Slytherin. Bagman is also unaffiliated with the Houses, though the yellow stars on his robes may indicate a slight Hufflepuff leaning, something that is indicated from the few glimpses into his character. McGonagall is the head of Gryffindor and shows her colors in a red Tartan. Percy, though once in Gryffindor is wearing navy blue. Percy has been noted for his studiousness throughout the books indicating, as the robes do, a possible place in Ravenclaw.
The next group of characters, the students, fits far more easily into this theory. Each one of them is wearing a House color. Ron, Parvarti, Padma, and Crabbe/Goyle are each wearing their House colors. Though we know to little about Parvarti and Padma to judge weather or not they could belong in a different House, Ron, Crabbe, and Goyle seem to represent the characteristic of their respective Houses.
This leaves us with the very last grouping, students who are not wearing their House colors: Harry, Hermione, Draco, and Pansy. Harry is wearing green, a Slytherin color. We know from the first book that Harry would have fit in well with the Slytherins. Hermione is in a blue, Ravenclaw Color; book five reveals that the Sorting Hat considered putting Hermione in Ravenclaw. This leaves Draco and Pansy. Draco is in black, a Hufflepuff color. Could Draco have some characteristics of Hufflepuff House? Pansy is wearing pink; a derivative of red and a Gryffindor color. Perhaps Pansy would have done well in Gryffindor.
Though this method of clothing and Houses is by no means exact, it does give some interesting insight into characters we rarely see. It is also interesting to note that Hagrid earlier on wears a yellow checked tie. Though the HP Lexicon lists Hagrid in House Gryffindor, he displays many Hufflepuff tendencies with his fierce loyalty to Harry and Dumbledore.