Brown Spot On Bottom Of Foot Under Skin. It can appear anywhere on the foot, including on the sole or under a nail. Reddish brown staining on our lower legs is often caused by problems with the veins on our lower legs.
They almost looks like stains and like they're under the skin. However, not all areas of discoloration on the skin are melanoma. Plantar warts may form on the bottom of a person’s foot if they have human papillomavirus.