Dark color of fired foot rims in earlier Chinese ceramics.

[peterp]

Chinese ceramics may have a dark or light-colored clay, or anything in between.
Kaolin clay usually is very white, gray and some other clays were yellow; others used during the Yuan and Song dynasties were brown, sometimes very dark, almost black.

Of course this depended on the clay available in the vicinity of the kiln. The dark color is the result of iron content in the clay. However, as dark clay in those times was used with some top porcelain wares. This color is often the subject to faking on newer imitations.
If an item that apparently has a dark clay body (foot rim) but edges, ridges, etc show no darker color, that usually means the clay of the foot rim was dyed. That again makes it likely that it is a later fake made with a light-colored clay.

Very few kilns actually did dye the unfired areas. The only one that comes to mind is the base of some early wares which turned orange-red/brown in color during the firing process, due to iron oxidation. But that looks different.

Sometimes we encounter items with a dark foot rim which shows a chip or two. That is mostly because someone wanted to check if the clay is dark inside.