Color of unglazed bottom in Chinese ceramics

[peterp]

How to decide what is fake and what is real, natural?

Chinese ceramics may have a dark or light-colored unglazed bottom or foot rim, or anything in between.
Kaolin clay is usually very white, some bottoms are grayish and some fired clays are 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.
If an item that should have a dark clay body shows white or gray abrasion or chips, that usually means an item was dyed. That means it may be a later fake made with a different clay than the original.

Very few kilns actually did dye the unfired areas. The only one that comes to mind is the base of Longquan wares, which may be orange-red/brown in color. However, that was not really dyed.
In this kiln usually an area of the bottom (ring-shaped or circular area) was wiped clean to remove the glaze before firing - for the item to stand on.
The wet cloth that was used caused iron oxidation of the unglazed clay during the firing process leading to an orange/reddish/brown color, which different from the actual color underneath the glaze.