What causes some eyes to change color by time of day or weather?

My eyes used to do this when I was younger and maybe 26 or so when I lived in another country I noticed they stopped changing by time of day. They stayed blue all three years I lived on an island, but when I left it back to the states for a week they started changing again - green morning, gray afternoon and blue evening. One summer in those 3 years I was in kyoto and they changed colors again. After age 30 my eyes began to gradually lose the sparkle and vividness they used to have. Every once in a while someone comments on my amazing eyes (marble like). I'm hoping with some dietary changes that maybe it will return.