It's a loss of pigmentation. Check out eye color on wikipedia to read up on it. You may also have some sort of genetic defect or possibly a problem with your eyes. If your eyes went from dark hazel to ice blue, you should probably consult an optometrist. If you have light hazel eyes (meaning green to blue) and they're staying blue, it could just be the season, your hair, or your skin tone changing which makes the hue look different. My eyes are dark hazel, and switch from a golden brown to an olive green on a regular basis, it's normally provoked by a make-up I wear, or if my hair lightens up. Sometimes if I've been crying a bit my eyes turn really green. It's all about how light hits them. Blue eyes need more light to see, so like I said, you may be experiencing an early sign of a vision problem and should have it checked out.
