It is almost inevitable that whenever you see a bull in frame, sooner or later that bull will see red. Almost always, this bull will see it on the person of the protagonist. And absolutely always, the mere sight of red will drive the bull into a frenzy. No exceptions. Bull sees red, you better start runnin'.
This is more popular in cartoons where the bull's emotions and behaviours can be drawn (gusts of breath snorted from the nose, red filling up the eyes etc.) and you don't have to pay for the pleasure of an irritated bull in your studio.
In real life, it's not the color of the cape the matador's waving; it's the fact that he's waving it. The fact that the color is irrelevant was proven long before the MythBusters tackled it on their show. Characters may also occasionally mention that bulls are colorblind, but this is also untrue (though the range of colors they can see is narrower than a human's). As this awareness has grown over time, expect to see many deconstructions and aversions, with the characters pointing out these significant bits of information.
The actual reason for the use of red in Real Life bullfighting is covered by Bring My Red Jacket. It's to mask the bull's blood.
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Lampshaded in a Russian ad, where a guy ends up in inside a fence with a bull. One of his friends shouts from outside he's wearing red. The guy covers himself with mud... and then another of his friends remarks "Aren't the bulls color blind"?
Anime & Manga
An episode of Naruto has our hero procure a ride on a bull, though the bull seems less interested in letting him ride than chasing that big, bright red spot on the back of his jacket. Enter two more bulls, and Naruto has a free run to wherever he wants to go (as long as he's doin' it on foot).
Despite not actually using bulls, an early episode of Pokémon still invokes the spirit of this trope. Scyther and Electabuzz are both said to fly into a berserk rage at the sight of the color red. It just so happens that Pikachu had discovered a love for ketchup in that same episode, so during a showdown between two rival gangs whose leaders used a Scyther and Electabuzz, a stray attack splits Pikachu's ketchup bottle open, splashing the stuff across both combatants' eyes. Since everything they see is now tinted red, they promptly start attacking everything in sight (including their trainers).
In Top 10, they mention that bulls are colorblind.
The same happens in a Donald Duck comic story, "The colorblind bull", where Grandma Coot has a - yeah, and said bull starts rampaging at the sight of anything that's light blue. Then a light blue hot air balloon appears...
Oh, the balloon was far from the worst part. The real disaster happened after the bull won the blue ribbon at a fair, and Donald (who still didn't understand what the problem was) got the bright idea of sticking the ribbon to the bull's forehead...
The best live-action example has to be from Pee-wee's Big Adventure, when the red shirt-clad bully chases our hero into a Texas rodeo.
Danny DeVito's character in Johnny Dangerously is whacked by handing him a Schlitz malt liquor and a red smoking jacket. Sure enough, the Schlitz Bull comes charging through the wall at him. Which, for those not aware, is a Shout-Out to Schlitz's famous late 70's/early 80's ad campaign where a bull would do a Kool-Aid Man wall crash at the end of each commercial.
Live Action TV
Busted in one episode of Mythbusters. The experiment culminated with a terrified Tory in a red jump suit standing very still in a rodeo ring with a temperamental bull. While rodeo clowns ran around distracting the bull, Tory was completely ignored. It did look for a time like he should have brought some brown pants
In the Adam West Batman, the trope is subverted when Batman escapes a deathtrap of being tied to the ground to be trampled by a herd of bulls. He frees himself and waves his cape to make the charging herd avoid him. When Robin notes he was able to do that without the cape being red, Batman tells him that cows are colorblind so the color of his cape didn't matter.
Even mythical bull-creatures are affected by this trope. In King's Quest VI: Heir Today, Gone Tomorrow, the hero manages to lure a minotaur into a trap by using a bright red scarf. The minotaur's growled comment: "Rrrrrreeed..."
Conkers Bad Fur Day had a boss battle against a bull that hated the colour red. And as Conker was a red squirrel...yeah.
Discussed in the secondAce Attorney Investigations when examining a particular object in the first case. Kay wonders if a bull would be drawn to Edgeworth because of his clothes, and he points out (correctly) that it's not the color that draws the bull's attention, it's the movement. She then retorts that, in that case, the bull would still be compelled to charge at Edgeworth because of his cravat.
In the casual adventure game Botanica, you need to dye a hang a piece of red cloth in front of a bull-like animal to make it charge. The cloth is white initially, so you also have to gather items to dye it red first.
The Classic Disney Short "For Whom the Bulls Toil" features a bull which only moves when Goofy produces a red handkerchief, with results that land Goofy in a Mexican bullfighting ring.
Woody Woodpecker encounters this moment in the early short The Hollywood Matador. It happens when the bull in question sees his red underside. (note: Woody's belly feathers were originally red)
Spoofed: In Scooby-Doo and the Cyber Chase, Fred uses Daphne's violet jacket to outmaneuver a charging lion, noting "it's not red, but it'll have to do". (In Fred's typical smartass fashion, he shouts "Leo!" because "Toro!" just provoked a "Where? Where?" reaction)
In the Disney short "Mickey's Rival", Mickey Mouse's animate car lures a bull away by waving its red turnlight.
In the Berenstein Bears cartoon this trope was averted when the characters are told to distract the bull by waving their brown scout hats. As the smart one points out, its not the color so much as the motion that the objects that makes them upset.
Space Jam had Daffy painting the rear end of one of the Monstars red. This elicited great laughter from the Toons in the audience, until the Bull from Bully for Bugs notices it, which is what Daffy was going for all along. Cue one Monstar going airborne involuntarily.
"Bully For Bugs" itself never mentions this trope - the bull is just angry.
An episode of Tiny Toon Adventures spoofing Star Wars features a bull named "Chewcudda". Never show Chewcudda anything red...not even your tongue.
Fridge Brilliance applies here: The bulls are, as stated above, robots. They were probably programmed to react that way to the color red because that's how a couple of kids assume bulls should behave.
The Popeye cartoon "For Better Or Nurse" has Bluto deliberately trying to injure himself (as is Popeye) so he can be near nurse Olive at a hospital. He tucks a red towel under his chin and taunts a nearby bull. Unfortunately for Bluto, the bull is smitten with the cow mascot of a dairy product ad billboard.
The Beatles: In the episode "Please Please Me, the Fab Four are having a picnic in Spain, and Ringo is shaking ants off their red picnic blanket. It attracts a bull, causing John, Paul and George to run.
George: Ringo, look out!
Ringo: Why? These little nippers can't hurt you! (after the bull passes and tears the blanket with its horns) Hmm. They wasn't ants after all. They was moths!
In The Simpsons episode "Pokey Mom", the family is at a prison rodeo when one of the contestants is in danger. Homer lifts up Lisa and uses her red dress to get the bull to charge towards him, but runs into a snafu when he needs "a little calming blue":