Spanish matadors began using a small red cape, or muleta, in bullfighting around the 1700s. Since that time, people have perpetuated the myth that the colour red makes bulls angry causing them to charge.
In a 2007 episode of the popular show MythBusters, the team tested this theory. They put makeshift matadors into an arena, each holding a flag of a different color, and wait for an angry bull to see red. The red, blue and white flags got equal, half-hearted attacks when they were motionless. In order to elicit an aggressive charge response from the bull, the flags had to be waved.
The conclusion: the color red isn’t what causes bulls to attack. Bulls don’t seem to have any color preference at all and will charge whichever object is moving the most.
Source: Discovery.com