That's what they are! Like the mood rings, that change color to reveal your mood, remember?
I don't know what other factors affect my eye color, and never really gave it much thought. But, they do change with my moods. And not just a little!
My eyes range from green, to blue, to hazel, to gray, and possibly even some combinations. However, they are never fully any one color. They may be blue, but there will be a green ring around the outside of the blue, or some similar combination. Other times, they may be mostly one color, except for the very center. Oh, and I have freckles in them, too.
I have always liked the fact that my eyes do change colors, and would be bored to have only one. I guess this is why they make colored contacts. So, do I prefer changing colors in my partner's eyes? Not really. In fact, up until I met my wife who's eyes do change between blue, green, and a blend of the two, I had always been drawn to brown eyes. Now, well I find her eyes to be so captivating, but I may have a slight prejudice based upon my feelings for her.
In the end, however, it's not the color of one's eyes that determines their attractiveness, but what lies below... The eyes are, I'm told, the windows to the soul.