Red is one of the primary colors of light. It's a primary color in additive color mixing and a secondary color in subtractive color mixing (the product of magenta and yellow). Pure red light is located at approximately 700nm. Red light accounts for 30% of the perceived brightness of an object. The pupil does not constrict in response to red light, which is why it is used when preservation of night vision is important (such as on military missions).

Subtractive (printed) red		Additive (computer screen) red

Psychologically, red is a powerful, active color. It can cause confrontations and excites the autonomic nervous system, raising the heart rate and causing heavier breathing. Red is highly noticable, which is why red cars are statistically more likely to be stolen or ticketed than vehicles of other colors. Red inspires quick decisions and spontaneity. It is also the "warmest" color. Restaurants use red in their decor because the color stimulates the appetite.