What has caused my eye color to change from dark brown to green?

If this is truly in the iris, it means that some of the pigment is being lost. This could be normal or associated with some forms of glaucoma. If it is only in one eye, this might be of concern. Some people get a gray/white ring on the corneas as they age (and we all age at different rates), making the iris appear lighter. Rarely, a Kaiser/Fleischer ring may occur on the cornea, making the underlying iris seem lighter in color. See your ophthalmologist for an accurate diagnosis.