Sometimes the nipples do change color a bit, but you are right that it would be relatively unusual for just one nipple to be changing color. Therefore, for this reason, I think it is worthwhile for you to get in to see your primary care doctor
or a dermatologist
about the issue. They will be able to examine the nipple closely to help you determine if this is something that you need to be worried about or not.
Make sure to mention to your doctor if you have any other symptoms, such as itching of the skin, scaling of the skin, or and discharge. It is relatively common, for example, to contract either contact dermatitis (an allergic reaction of the skin to some chemical like a soap or cleaning product) or eczema
(excessive dryness of the skin). These could definitely occur just in a single patch on one breast. In addition to changing the color of the skin, they should itch or flake, however.
Similarly, a fungal infection of the skin would be another potential cause of changing skin color in a single patch. Finally, if there was a mole near the nipple before, it would be important to make sure this is not a changing mole, as that could be a sign of a skin cancer.
Get in to see your doctor right away!