Hello, the discoloration of the foot pad is generally a physical phenomenon, which has little significance and cannot fully reflect the physical condition. If it is black on both sides and white in the middle, there are two possibilities:

1, with more powder edges and less middle, and no discoloration.

2, the foot has a depression, the foot sticker is not firm, the middle is empty, and it is not attached to the skin, so it does not change color.

I hope I can help you.