I recommend that your see your primary doctor
or your ophthalmologist
(eye specialist). Vision is a exquisitely balanced sense that can quickly and irreversibly be lost if there is a disease process. As such, seeing your doctor regarding any concern of your eye is important.
The retina is the inner part of the eye that holds the nerves and allow the transition of the light signal to the brain--thereby giving us sight. The retina lines the inner part of the eyeball. As such, it is unlikely that your are seeing change in color of your retina (this requires a specific instrument to see). The two most visible part of your eyes are the pupil (which is the colored part of your eye) and then the sclera (which is the white of the eye).
Darkening of the sclera can occur because of a couple of conditions (if this is in fact what you are referring to). The most common reason for darkening of the eye is actually liver disease as the bile salts build up in the blood and stain the sclera. This is known as scleral icterus. If there is a discrete spot, this can be a hematoma--or a contained ball of blood. Hematomas can form from minor trauma resulting in a rupture of the small vessels in the eye. This often heals itself. There are other less common causes.
If you are in fact referring to retinal pigmentation--then there are a few causes. Retinosa pigmentosa is a genetic condition seen in children. Other problems such as macular degeneration, diabetic disease, hypertensive disease, and even retinal cancer (retinoblastoma) can cause this.
I would recommend seeing your primary doctor.