The subtraction of a color equals the addition of it's opposite. For example, subtracting yellow means an increase of blue. In addition, each color is made up of equal amounts of the two on either side, e.g. green is formed by combining cyan and yellow, and blue and red mix to make magenta. The amount of the various colors within a continuous spectrum of light is measured as color temperature . Color temperature is expressed on the Kelvin temperature scale.
Artificial light is almost never white.
Our brains remember what things are supposed to look like, and are quite adept at correcting our perception to view the primary light source as white even if it isn't. Film, however, cannot make this adjustment on its own. For this reason, we must at some point compensate for film's limitations. For color negative film , this adjustment can all be done in the darkroom if necessary. Daylight film is the most commonly used, as it is designed to give an accurate representation of a scene in daylight.