Color or colour (see spelling differences) is the visual perceptualproperty corresponding in humans to the categories called red, green, blue and others. Color derives from the spectrum of light (distribution of light energy versus wavelength) interacting in the eye with the spectral sensitivities of the light receptors. (Wikipedia)
Eye cones are special machines too. They are made to see three different colors: red, green, and blue. When the color blue goes into the eye, the blue cone tells the brain, "Hey, that color is blue." Because we have only three different colors of cones, we aren't able to see all colors. Ultraviolet light is a color that isn't able to be seen, for example. Some animals and insects have more cones and can see colors that we can't see. People who are color blind have damaged or missing cones.
The surface of seas and lakes often reflect blue skylight, making them appear bluer. The relative contribution of reflected skylight and the light scattered back from the depths depends strongly on the observation angle.
Blue has qualities that oppose red (Birren 260). Studies show that its presence helps lower blood pressure and pulse rate. This may seem to be solely positive responses — and they are, in certain situations — but blue also retards the body’s reactions and hormonal activities. The immune system doesn’t heal wounds as fast due to the slower metabolism and response time. Thus, medics used to wear red clothing and wrap wounds in red cloth to expedite healing because that is the stimulant color.