Why Did One of My Dog's Eyes Turn a Lighter Brown?

A change in iris color in one eye is worth having your vet look at, even when the eye seems comfortable. A lighter or patchier iris can come from iris atrophy, a thinning of the iris that's common in older dogs. Changes in iris color can also be a sign of inflammation inside the eye (uveitis), which isn't always obviously painful at first.

Your vet can check the eye pressure, look for inflammation, and compare the pupils in bright and dim light. Uveitis and glaucoma can damage vision if they aren't treated early, so it's better to rule them out. If it turns out to be a harmless aging change, nothing needs to be done.

In the meantime, go sooner if the eye turns red, cloudy or watery, if she squints or avoids bright light, or if the pupils look different in size.

See iris atrophy and dark spots and color changes in the eye.

This is general information, not a diagnosis. Please follow your own vet's advice.