Why do my eyes change color?

Well, my eyes, since birth, have been considered blue, and they are blue, but they change colors over a short period of time sometimes, ranging from a super light blue, to a deep blue, to a bluish green. Sometimes I'll look at my eyes in the mirror and they'll appear nearly identical, but two different colors. Honestly, I don't know what causes that, and it doesn't bother me much, but when I look in the mirror it just really makes me wonder what causes that. I mean, it's not like its some kind of disease or something (I hope XD) but I was just wondering. Ok, thanks :3