When we think of color of light we usually think of "white" light. Light is considered white when it contains a roughly even mix of of the three primary colors: red, green and blue (the infamous RGB). This combination of light colors is perceived by the human brain as colorless.

Although the human eye can detect very small changes in the color mixture of that "white", the brain refuses to admit there is a difference; as long as there are reasonable amounts of those primary colors, our brain will say that "the light is white".

Film cannot make that automatic compensation to adjust the color of the white light "seen" by the brain. Photographers have to pay attention to the differences between "white" light sources. To classify variations in the color of white light, photographers borrowed the color temperatur scale from Physics. This scale is based on the fact that, in a vacuum, when a material is heated to a high enough temperature, it will glow with a color that depends of the temperature. This temperature is measured in degrees Kelvin which is a unit of temperature used in scientific work (the absolute zero is 0°Kelvin).

Since it is based on the heating of a material, the high color temperatures have a high proportion of what we call "blue colors": 10.000 K is a bluish light; 2000 K is a red to yellow light.

There are three standard light color temperatures used in photography: 5500 K, usually called daylight, and two tungsten color temperature standards, 3200 K and 3400 K. One can buy color balanced film for any of those color standards.