At five o'clock, when Vernon leaves work, he bumps in to a man wearing violet (shade of purple) robes.
Quirrell's turban is purple.
Hagrid swishes his umbrella down and with a flash of violet light, Dudley sprouts a curly pig's tail
In Ollivander's window, a single wand sits on a faded, purple pillow.
Dumbledore wears a purple cloak.
There's a girl in Harry's class named Lavender Brown. Lavender is a shade of purple.
Harry sees enormous purple toads in the Magical Menagerie.
Dedalus Diggle's robes are purple.
Harry's letter from Hogwarts is sealed with purple wax.
During the train ride to Hogwarts, the sky is described as being "dark purple."
The fire engulfing Harry and Hermione in the room with the potions is purple.
At the end of Chapter 2, Harry recalls "a tiny man in a violet hat bowing to him once while out shopping."
Harry recalls "that a bald man in a very long, purple coat had actually shaken his hand in the street..." Thanks to Steph
Chamber of Secrets
At Flourish and Blotts, Lockhart's photographer's camera emits purple smoke every time a picture is taken.
At one point, Ron's broken wand emits purple bubbles.
Lockhart's favorite color is lilac, a light shade of purple.
Fudge is wearing purple boots.
Lockhart wears plum-colored robes to the Dueling Club.
"London was soon far behind, replaced by neat green fields that gave way in turn to wide, purplish moors."
When Filch gets sick his nose turns "unusually" purple.
Young Mandrakes are tufty little purplish green plants.
Neville buys a pointed purple crystal, among other things, to protect him from the heir of Slytherin.
When McGonagall comes out onto the Quidditch field to announce that the game has been cancelled, she comes out with an enormous purple megaphone.
Filch's Kwikspell letter is purple.
Prisoner of Azkaban
The sleeping bags the students slept in are purple.
The Knight Bus is purple.
Stan Shunpike is wearing a purple uniform.
Lee Jordan uses a large, purple megaphone to commentate the game.
As Harry, Ron, and Hermione leave Hagrid's hut, the sun was setting and the sky "turned to a clear, purple-tinged grey."
Goblet of Fire
The letter Mrs. Weasley sent to The Dursleys was purple.
After eating the Ton Tongue Toffee, Dudley's tongue swells and turns purple.
Ron gives Dobby a pair of violet socks Mrs. Weasley had made for him.
The bag that holds the miniature models of the dragons is purple.
The programs at the Quidditch World Cup are purple.
A lady in a portrait who hangs out with the Fat Lady is called Violet.
Mr. Roberts is described as having deep purple shadows under his eyes.
A group of haggard-looking Ministry wizards rushed past Ludo and the Weasleys, pointing at the distant evidence of some sort of magical fire that was sending violet sparks in the air.
At the camp site, wizards in white long robes sat together roasting a rabbit over a blazing purple fire.
When Harry looked at the ceiling that was enchanted to look like the sky outside, he noticed that it was very stormy looking, and the clouds were black and purple.
The words the Omnioculars' commentary system uses are purple.
The stairs are covered in a rich purple at the Quidditch World Cup stadium.
The chairs in the Top Box of the Quidditch World Cup stadium are purple.
Rita Skeeter wore a furry, purple-colored shirt.
Rita Skeeter's photographer's camera emits purple smoke with every flash.
Ron went purple when Fleur Delacour came to the Gryffindor table to borrow the bouillabaisse.
The liquid that Madam Pomfrey pours on to Harry's cut after he faces the Horntail is purple.
Cornelius Fudge is wearing a purple hat.
Ludo Bagman wore a purple robe with yellow stars.
One of the taps in the prefect bathroom gushes out purple bubbles.
The Sleeping Potion Harry took after his encounter with Voldemort was purple.
Order of the Phoenix
After Harry heard the loud crack while outside the Dursleys', two large purple hands reached through the open window and closed tightly around his throat.
And then Aunt Petunias thin, horsey face appeared beside Uncle Vernons wide, purple one.
Harry stepped into the shadow of a large lilac tree and waited for Dudley.
Several more references to a purple Uncle Vernon: purple-faced, shouting, his fist still raised; the vein in his purple temple was throbbing worse than ever; Uncle Vernon seemed to be swelling, his great purple face stretching before Harrys eyes;
...his massive purple face coming close to Harrys ...
Tonks was described as having short spiky hair that was a violent shade of violet.
Dedalus Diggle dropped his violet-colored top hat.
Mundungus rescued Ron from an ancient set of purple robes that had tried to strangle him.
Fred and George invent 'puking pastilles' to sell to students - the orange half makes a student throw up to get out of class; the purple half restores them to good health after leaving the class.
Mrs. Weasley wore a quilted purple dressing gown the morning of Harry's hearing.
In the Ministry of Magic, Harry noticed several pale violet paper aeroplanes in the lift.
Fudge turns a shade of magenta 'of which Uncle Vernon would have been proud.'
Fudge also turns a slightly deeper shade of puce (which is a dark brownish shade of purple for those of you reaching for a dictionary).
Mrs. Weasley places a pair of maroon socks on Ron's pile of clothes.
Tonks wore a purple hat shaped like a pork pie while accompanying Harry to the Hogwarts Express.
At the staff table, Dumbledore wore deep-purple robes scattered with silvery stars and a matching hat.
Filch came wheezing into the room. There were purple patches on his sunken, veined cheeks.
Dumbledore was wearing a magnificently embroidered purple and gold dressing gown over a snowy white nightshirt when Harry came to tell him about the snake biting Mr. Weasley.
Gilderoy Lockhart was wearing a long lilac dressing gown in St. Mungo's.
A dull purple flush was creeping up Neville's plump face when he unexpectedly met Harry, Ron, Hermione, and Ginny in St. Mungo's.
Again we meet the purple Knight Bus and Stan Shunpike in his purple uniform.
Dolores Umbridge's face turned an ugly, patchy violet when she read Harry's interview in The Quibbler.
Marietta Edgecombe's face was horribly disfigured by close-set purple pustules that spelled out SNEAK.
One of the Fred and George's fireworks was a large, smoking, purple bat.
Neville was turning steadily more purple while Crabbe overzealously restrained him in Umbridge's office.
Neville had a large purple lump swelling above his right eye after escaping Umbridge's office.
The sky was turned a light, dusky purple littered with tiny silver stars.
A Death Eater struck Hermione with a streak of what looked like purple flame.
Tonks was wearing a bright purple T-shirt bearing the legend, "The Weird Sisters".
And once again, Uncle Vernon turned a deeper shade of puce, this time while glaring at Mr Weasley.
The Ministry of Magic posts and distributes purple posters and leaflets with security advice and photographs of Death Eaters.
Vernon Dursley is wearing a puce dressing gown when he opens the door to Dumbledore.
Horace Slughorn first appears wearing lilac silk pajamas.
Hermione ends up with a brilliantly purple black eye after an encounter with Fred and George's telescope.
Fred and George's shop has a gigantic purple poster in one window with flashing yellow letters about 'U-NO-POO.'
Ginny's new pygmy puff, Arnold, is purple.
Dumbledore wears a 'flamboyantly cut suit of plum velvet' in his Pensieve memory of going to see Tom Riddle at the orphanage.
A third-year girl delivers two scrolls of parchment tied with violet ribbon to Harry and Neville.
Harry's Draught of Living Death turns the exact shade of lilac described by the textbook.
Hermione's Draught of Living Death is a deep shade of purple.
A great bang and a puff of purple smoke cleared to show that the Heads of House were able to reunite Susan Bones with her splinched leg.
Mundungus was slowly turning purple after Harry pinned him against a wall by the throat for stealing Black family items.
Ernie MacMillan invented his own potion, which had curdled and formed a purple dumpling at the bottom of his cauldron.
The merpeople have purplish hair.
What appears to be Dumbledore is wrapped in purple velvet with golden stars.
Other Purple References
Many times throughout the books, characters (especially Vernon, Petunia, and Ron) are described as being purple in the face over something.
JKR showed up to the movie premier in a purple dress.
The Hebridean Black Dragon's eyes are purple.
The Graphorn has purple horns.
Pride of Portree -- a professional Quidditch team -- has purple robes.