There are a few different potential causes for your gums turning brown.
First, it could be that this is a normal finding for you. It turns out that, as people age, the color of their gums also sometimes changes, particularly in certain ethnicities. This is not, generally, a serious medical problem or a sign of an underlying condition, as long as the teeth themselves and the gum line near the teeth are healthy.
Gum discoloration can also occur when there are tooth infections or when their is gingivitis (inflammation or infection of the gum line). However, this doesn't seem likely in your case, because you specifically mention that your actual gum line is health and that you have an active dental hygiene regimen.
Finally, gum darkening may be caused by ingesting certain food stuffs, especially lots of coffee or tea. Smoking also can darken the gums.
I recommend that you mention this issue to your dentist
or your primary care doctor
. They can take a close look and make sure there is no evidence of tooth infection or gingivitis, or of any other condition that would require treatment or further medical evaluation.