Yes, it is possible that the eye color of adults can change, and it can also only happen to one eye. It is then called Heterochromia. There are two possibilities for different colored eyes, either congenitial (genetic) or acquired. Since you ask only for eye color changes in adults, we can skip here the genetic reasons, as these show up in kids already. If you are interested in them, have a look into the Wikipedia article on Heterochromia.The basis for our eye color is genetically determined, changes can later occur due to diseases (so if one experiences a change in eye color, it is always a good idea to consult a doctor) and also due to medications.The eye undergoing a change in pigmentation can either lighten up or darken.
