What would cause eye color to change seasonally?

In the summer they are green and in the winter they turn brown. Right now they are sort of half-way. The outer part is green and the inner part is brown. The colors are very different and distinct. There really isn't any mixing of the colors in between. It freaks people out sometimes.What would cause eye color to change seasonally?
my eyes do that, change from green to brown with the seasons, never worked out why, might just be the changes in levels of light, or like how your hair gets lighter in summerWhat would cause eye color to change seasonally?
well my eyes have never changed seasonally, but they do change with my emotions, sometimes they are more green than brown, but when I'm tired or emotional they turn very very green, and then they are a golden brown most of the time, sometimes varying to a very dark brown, I guess it could be allergies or something like that, but it's very cool to have color changing eyes, congrats!
hazel eyes is my best guess..?