As you learn more about digital photography, you're bound to run into a measurement scale for color that seems a bit odd. It may seem unusual, but the color of visible light is actually measured by temperature using degrees Kelvin.
Lord William Thomson Kelvin (1824 – 1907) was a British physicist who introduced us to the absolute scale of color temperature. It’s based upon a “black-body radiator” that is heated and as the temperature is increased light becomes visible beginning at the red end of the spectrum. As the temperature increases, the color moves from the red end to the blue end of the spectrum.
Color temperatures range from about 1,700k for the yellowish red color of a flame from a match to about 10,000k for a partly cloudy sky. Daylight is considered to be about 5,500k.
For most digital photographers, you won’t need to know color temperatures when working with your cameras controls. Many advanced cameras and digital editing software do, however, have color temperature controls that are based upon the Kelvin temperature scale.
An interesting sidelight to color is the difference between the Kelvin temperature scale and our emotional response to color. Colors that are at the red end of the spectrum have always been considered warm colors. These are the reds yellows and oranges. Yet on the Kelvin scale, these are the color with the coolest color temperatures. Inversely, the colors that we’ve always considered cool colors, such as blues, have the highest color temperatures.