White Balance And Color Cast
Light has color.
Most people don't realize what difference light sources make in photography until they see the results--and then wonder why the picture looks absolutely nothing like what they remember pointing the camera at.
Light of various temperatures is associated with a temperature in Kelvin. For example, a candle is about 1500 K, a standard light bulb about 3400 K, and the flash on your camera about 5600 K. Light from a sunset, for example, is a rich golden yellow. Light from flourescent bulbs actually shows up as purple in some photos! If you really want your subjects to look like people and not refugees from a planet of purple-skinned strangers, you'll need to keep White Balance in mind.
White Balance is an automatic setting on most digital cameras to account for these adjustments in light sources. Cameras come with a variety of pre-programmed settings. The camera recognizes that if the setting is for flourescent bulbs, then "white" is actually going to look "purple." It will find an example in the frame that it thinks is supposed to be white, and adjust the spectrum for the picture accordingly. Then the picture will turn out with the colors that people expect to see. Unless your picture has wide areas of just one color, it's generally safe to let the camera decide the white balance automatically.
But, if your camera didn't do the white balancing job correctly, then you'll get purple-people syndrome, and you'll have to use your photo-editing program to make up for what the camera missed. Most programs have some sort of color balance control, and the majority of them work the same way the camera was supposed to. Select a point in the picture that was supposed to be white, and the computer will then adjust the entire color spectrum of the picture approprately. Presto, no more purple people.