Color correcting Photographs

1. A photograph has a color cast if it contains a color tint that misrepresents the true color of the objects in the original scene. For instance, sunlight can add a blue color cast, incandescent or tungsten light produces a red/orange cast, fluorescent light produces a pink, blue, or green color cast depending on the bulb, old photographs tend to fade to a yellow cast, a projector typically gives a purple cast, some scanners can create a color cast, and photographs produced from film can have a cast from improper chemical development.
   
   
2. Both the Color Balance and Levels adjustments are good for removing a color cast or otherwise altering color. Color Balance is preferable for the uncommon situation where the image has a different color cast in the highlights than in the shadows. Such a color shift can happen as a photograph ages. Otherwise, Levels generally gives better results and also allows you to adjust tonality at the same time.
   
   
3. In the Color Balance adjustment, the Preserve Luminosity option attempts to keep the tonal values of the image unchanged. For example, if the red channel is lightened, the blue and green channels are darkened to compensate. Since we typically want to avoid these tonal shifts, we usually want to check this option.