A stimulant, red is the hottest of the warm colours. Studies show that red can have a physical effect, increasing the rate of respiration and raising blood pressure.
The expression seeing red indicates anger and may stem not only from the stimulus of the color but from the natural flush (redness) of the cheeks, a physical reaction to anger, increased blood pressure, or physical exertion.
Red is power, hence the red power tie for business people and the red carpet for celebrities and VIPs (very important people).
Flashing red lights denote danger or emergency. Stop signs and stop lights are red to get the drivers’ attention and alert them to the dangers of the intersection.
In some cultures, red denotes purity, joy, and celebration. Red is the color of happiness and prosperity in China and may be used to attract good luck.
Red is often the color worn by brides in the East while it is the color of mourning in South Africa. In Russia the Bolsheviks used a red flag when they overthrew the Tsar, thus red became associated with communism. Many national flags use red. The red Ruby is the traditional fortieth wedding anniversary gift.
Information from http://desktoppub.about.com/cs/colorselection