Just put them on, Archie, there's a good chap. You can't walk around like that, the Muggle at the gate's already getting suspicious..."
-- unnamed Ministry wizard telling Archie NOT to wear a flowered nightdress
"Fine. Go naked. And, Harry, make sure you get a picture of him. Goodness knows I could do with a laugh."
-- Molly Weasley, when Ron refused to wear his second-hand dress robes
Traditional wizard clothing consists of robes, cloaks, boots, and possibly a hat of some kind. The colors are bright and and the style somewhat extravagant. (Lockhart is a noteworthy example of this.)
Students of fourth year and above are required to have dress robes to wear to formal events.
Harry: much like his school robes, but bottle green (bought for him by Molly using his money)
Ron: (bought second-hand by Molly, so she didn’t have many choices; frightfully out of style) maroon with lace at the collar and cuffs, which he tried to remove with a Severing Charm.
Crabbe and Goyle: green (Harry thought they looked like moss-covered boulders)
Parvati Patil: shocking pink
Padma Patil: bright turquoise
Fleur Delacour – silver-gray satin
Draco Malfoy – black velvet with a high collar, which in Harry’s opinion made him look like a vicar
Pansy Parkinson: very frilly robes of pale pink
Hermione: floaty, periwinkle-blue material
McGonagall: red tartan and a rather ugly wreath of thistles around the brim of her hat
Ludo Bagman: bright purple with large yellow stars
Madame Maxime: flowing gown of lavender silk
Percy Weasley: brand-new, navy-blue dress robes
Dumbledore, when he arrived in Privet Drive, was wearing long robes, a purple cloak that swept the ground, and high-heeled, buckled boots (PS1)
"people in cloaks" that Vernon spotted on his way to work: emerald green, violet (PS1)
robes have pockets, inside and out (e.g., CS13)
Cornelius Fudge wears a pinstriped cloak, a violet top hat, emerald green trousers, etc.
Ear muffs are worn (Dumbledore remarked that he had blushed when Madam Pomfrey had complimented him on his pair (PS1)), and not always in winter: Sprout provided them for her second-years to protect their ears from the cry of the Mandrake. These must have been enchanted, since normal earmuffs don't block sound entirely (CS6).
Rita Skeeter wears acid green robes, against which her Quick-Quotes Quill blends perfectly (GF20)
Neville's grandmother wears a long green dress with a fox-fur scarf, a tall hat with a stuffed vulture on it, and a big, red, handbag (PA7).
Young wizards and witches may affect dragon hide accessories to look cool, while the rest of their clothing may be somewhat similar to that of Muggles of the same age. Bill Weasley wears dragon hide boots, and the rest of his clothing "would not have looked out of place at a rock concert" (GF5). After the twins left Hogwarts, they bought themselves dragon hide jackets, apparently Common Welsh Green since they were lurid green (OP38). Tonks when dressed as a Muggle to meet Harry at King's Cross Station wore clothes that could pass as Muggle clothes, but the T-shirt bore the name of a wizarding musical group, so evidently young wizards and witches wear that sort of thing among themselves (OP38).