Have you noticed how light has different colors? A lightbulb is different from daylight and noon sun is different from sunrise for example. After you’ve worked with photography for a while, you’ll notice that light seems to have different colors based on different times of day, weather conditions, and whether or not it is natural or artificial light. These color differences are referred to as light temperature or color temperature. The Kelvin scale measures the amount of heat reflected by different light sources under controlled conditions. These amounts are then applied to light to give us a scale of the temperature of light colors. These temperatures are not exact in real world conditions but they give us a constant (language) to use so that everyone is working with the same information.
The Kelvin scale used in photography ranges from red to white to blue. This only covers a visible light spectrum and not the entire light spectrum into the ultrareds and ultraviolets. An easy way to remember the progression of light colors is the acronym Roy G. Biv. This stands for red, orange, yellow, green, blue, indigo, violet. It doesn’t exactly match the way light colors behave in photography but it is a decent memory technique to remember the general sequence of the colors and temperature progression. Green in particular only shows up occasionally in lighting tint and is replaced in lighting conditions by white, or no tint.
Another difference is that what we think of as hot and cold are backwards in light temperatures. Red is actually cooler than blue when the temperatures are measured. However, probably because of a natural association of red with fire, red tinted light is called warm and blue tinted light is called cool by most photographers. So why do you need to understand color temperatures? A lot of the better photo editing programs use a temperature scale in their adjustment sliders for correcting white balance. Understanding these temperatures will also make it easier for you to select the correct filters and light sources for your photography as these are often labeled with Kelvin degrees as well.