If you are a heavy smoker, your whites of the eyes may get a yellow tinge from the smoke. If it is just small areas become yellowed, it can be a sun exposure problem on of the eye. But the most common reasons for the white part of our eyes to turn yellow are a symptom of liver failure. I suggest you to see your doctor timely.
As your situation is not very serious, I think there are several possible causes of iris color change. Maybe it is just the inflammation inside the eye or the loss of the some of the pigment. But in order to prevent the happening of some diseases, you'd better check it with a simple blood test. But if the entire white part of your eyes is turning yellow, elevated bilirubin in your blood may happen. There are some condition which affects the liver and gallbladder.