Fortunately, the serious medical problems that cause yellowing of the eyes, such as problems with the liver, should cause equal yellowing of both eyes as well as yellowing of the skin. So the fact that this is just affecting one of your eyes is a good sign, suggesting that you are less likely to have a serious medical problem. Nevertheless, you should get this checked out by either your eye doctor
or your primary care doctor
One possibility would be sun damage to the eyes. Sun damage can occur to the eyes just like to the skin. In the case of the eyes, it can lead to darkening of the whites of the eyes, imparting almost a yellow color.
Another possibility would be a mole in the eye. Moles are collections of pigmented cells in the skin, and they can also occur in the whites of the eyes. If you have developed a new mole in the eye, that would need to be examined by your eye doctor to make sure it is not a cancer.
Finally, a pterygion is a thickened area of sometimes yellow tissue at the corner of the eye that is caused by chronic inflammation of the surface of the eye.
Talk to your doctor as soon as possible!