Left: Warm Grey. Right: cold grey
Color temperature is a characteristic of visible light that has important applications in lighting, photography, videography, publishing, and other fields. The color temperature of a light source is determined by comparing its chromaticity with that of an ideal black-body radiator.
Higher color temperatures (5000 K or more) are "cool" (green–blue) colors, and lower color temperatures (2700–3000 K) "warm" (yellow–red) colors.
As the sun crosses the sky, it may appear to be red, orange, yellow or white depending on its position. The changing color of the sun over the course of the day is mainly a result of scattering of light, and is unrelated to black body radiation.
For colors based on the black body, blue is the "hotter" color, while red is actually the "cooler" color. This is the opposite of the cultural associations that colors have taken on, with "red" as "hot", and "blue" as "cold". The traditional associations come from a variety of sources, such as water and ice appearing blue, while heated metal and fire are of a reddish hue. However, the redness of these heat sources comes precisely from the fact that red is the coolest of the visible colors, the first color emitted as heat increases.
In the desktop publishing industry, it is important to know your monitor’s color temperature. Color matching software, such as ColorSync will measure a monitor's color temperature and then adjust its settings accordingly. This enables on-screen color to more closely match printed color. Common monitor color temperatures, along with matching standard illuminants in parentheses, are as follows:
5000 K (D50), 5500 K (D55), 6500 K (D65), 7500 K (D75), 9300 K.
Note: D50 is not a color temperature. It is scientific shorthand for the daylight spectrum at about 5000K.