Is it a concern that my eye color has changed at 59?

The color of you eye is the color of your iris as seen through the cornea. The change in your eye color is either cause by a change in the color of the iris or the clarity of the cornea. Perception of eye color is often variable with the observer. Some might call one pair of eyes green and another may say hazel or blue. Rarely external factors such is the color of the light in the room can confuse patients that their eye color is changed.
The cornea is a clear dome over the front of the eye that is usually quite clear as a youth. With aging or high blood lipid levels its clarity may change causing a cloudy appearance that the patient or observer may call "gray." Hence a brown or blue eye may turn gray. A scarred or swollen cornea also has a gray appearance. The color of the iris behind an abnormal cornea is not changed.
Iris color rarely changes. Iris inflammation, atrophy, tumors, abnormal blood vessels or surgery can change the iris appearance. Some glaucoma drops can change iris color. Anytime the appearance of an eye changes, it should be examined by an eye doctor. It is always better to be safe.

Eye color is related to the color of the iris.  This may be blue, brown, green or hazel.  Typically the iris color stays fairly stable over the years.  If one notices an assymetric change of color, say one eye seems different than it used to be, or if there is a sector of color change this should be brought to the attention of an ophthalmologist. While not very common, changes such as these can represent tumors and are best addressed at the earliest possible time.  A change in the pupil shape would also be important to note and would be another indication for further evaluation in this setting. There are some glaucoma medications which cause changes in eye color.  Again, if you notice something different it is wise to bring this to your eye doctors attention.