The color red provokes a stronger emotional reaction in people than any other color. Things designed to attract attention – like stop signs, warning signs, and the cape waved in front of a bull – are very often colored red. Why red, and not a different color?
I think the reason is as follows. The three primary colors are red, green and blue. Green and blue are very common in nature – they are the colors of plants and the sky, respectively. Red, in contrast, is rare. Since red is the only primary color we don't see all the time, it is effective at attracting our attention.