I believe it APPEARS that your eyes change color, but they don't REALLY do this. It's because light hits your eyes just right and it can make it LOOK like your eyes have changed color, but they haven't. It's a strange phenomenon that happens to a lot of us. When light hits my eyes in a certain way, they appear to change color, as well. Don't worry about this, Mortimer, it IS quite normal. Nothing is wrong with your eyes.