The reflection that you see when a light is shone into your pup's eye is a reflection of light back from the retina and the tapetum. These are normal parts of the back of the eye that reflect light and send information from the eye to the brain. In light, red or white colored dogs there tends to be less pigment in the eye too, and so they will look red because of the blood vessels in the eye. In darker colored dogs the reflection may look green.
What you are describing, a gray color instead of the normal shine back, could be a cataract.
A cataract is a lens that has abnormal conformation such that rather then light going through the lens and being focused on the retina so that information can be sent to the brain, it reflects back the light, thus the cloudy gray appearance.
Cataracts can be due to diabetes, trauma, old age or they can be inherited (unlikely if she is older). As long as some light is getting through she can see shadows enough and they may not affect her getting around, and if only one is affected she may be relying on her "good" eye. See the link below for a picture of a dog with a cataract:
The other possibility is an old age change whereby the lens looks blue or gray in color simply because it gets thicker with age. These dogs, although their lens looks grayish, have a normal retinal shine when light goes into the eye and can see just fine. Usually both eyes will look similar with nuclear sclerosis. See this picture of a dog with nuclear sclerosis:
Notice with nuclear sclerosis that although there is a slight blue or gray tinge to the lens it isn't foggy and light is penetrating the eye.
I highly recommend that she see her veterinarian for an examination. They can differentiate between a cataract and nuclear sclerosis. If this is a cataract they can run some tests to try and determine why it formed or refer you to a veterinary ophthalmologist to perform more tests.
Another possibility, seen less commonly, that can occur in dogs with hypertension (high blood pressure) is bleeding into the eye and a detached retina. That would need immediate treatment to save her vision.
Best of luck with her, please let me know if you have any further questions.