Iris can change when there are some kinds of diseases or aging of the human beings. But the procedure is a slow and permanent. It will not happen overnight or go back and forth. Iris color can change when you are in different circumstances or wearing different color of clothes. But it is due to the reflection of the lighting and color of clothes you are wearing.
I also heard that the eye color will change. But I don't believe it. Maybe the guys just have some disease so that his or her eye color changes. And I also heard that there are at least two genes control eye color and usually the genes work in complicated ways. Is there any relation with the genes? Maybe there are more genes involved in this process.