There seem to be various conditions that affect the color of the outer edge of the iris, such as:
and I imagine there are others.
Generally they can indicate an underlying medical condition (e.g. the Fleisher ring is indicative of Keratoconus).
The limbal ring is said to indicate good health, but I couldn't find any hard citation for this:
What is a list of all such rings, and methods for distinguishing between them?