The iris also has pigment cells called melanocytes.  These are the same kind of cells that create pigment in the skin.  The pigment in the melanocytes is brown.  When there are small amounts of the pigment the color is more yellow than brown.

The pigment is located on the top of the iris and the blue muscle is on the bottom.  When there is a small amount of pigment, the yellow and blue create a green or hazel colored eye.  The more pigment, the more brown the eye becomes.  In addition, most eyes will have freckles that will add more brown color to the eye.

So, human eyes tend to be blue, hazel or gray.  Animals have different colored muscles and pigment resulting in different color combinations.  One example is the rabbit, which can have pink eyes.

Does the eye color have anything to do with the health of the eye?

Yes, the eye color can be affected by several conditions.  One such condition is heterochromia, which is different colored eyes, usually one blue and one brown.  In the congenital form of this condition, one eye becomes inflamed and loses its pigmentation and appears lighter in color.  It is also at risk of developing glaucoma.

Sometimes if there is trauma to the iris the pigment can be released and cause the eye to look darker.  Certain tumors can increase the brown pigment in the eye.  Some medical conditions can cause iron deposits in the eye and a rust coloring of the iris.  In all of these conditions the iris is usually darker.

There is also a condition called albinism that results in lighter colored eyes.  This is because of a genetic defect in the production of pigment.  This affects the pigment in the iris as well as in the skin and hair.

Medications such as some glaucoma eye drops can cause an increase in pigmentation.

But, if you want to change your eye color, just wait.  As we age, the eyes slowly lose pigment and become lighter.  So, as the brown eye lightens it just may become green.