Why does a baby have one red eye and one green when the flash goes off for a photo?

Red eye refers to the tendency of the pupil of the eye to glow red in flash Photography. It is caused by light from the flash reflecting back from blood vessels in the back of the eye. The color goes green when the angle of refraction changes. It is not unusual to see one green and one red eye. The effect is worst in dim light (when you naturally want to use flash) because the pupil is dilated (expanded) more in dim light and more light enters the eye.

Red eye is a common problem with small cameras that have the flash close to the lens, especially when photographing babies, pets and people with blue eyes, although it can occur at other times as well. A similar problem called "green eye" can occur along with, or instead of, the red, and is most common with pets, especially cats. Small cameras have no place else to put the flash, and most manufacturers attempt to minimize the effect by multiple flashes, the theory being that the first flash will cause the pupil to contract, and the next one will illuminate the subject for the image exposure. This is reasonably effective under some circumstances, but generally leaves a lot to be desired.

The only real cure is to shoot profiles, increase the light in the room to contract the pupils, or use a flash that is positioned some distance away from the lens. Most photo editing programs such as google's Picasa 2 (free download) have a red eye correction tool, and that is often the easiest way to deal with the problem.