Most likely answer: something to do with the perception of the color, but not the color of the eyes themselves. The eyes are not changing. People perceive the color to be different in different lighting conditions due to the way the brain processes colors. The general area of vision science concerning this is called color constancy. The basic idea is that colours do not remain constant when either lighting conditions or surrounding colors change. You can find out more at the Computational Vision lab at Simon Fraser University.