The lack of oxygen in the skin makes the skin look green. It is the characteristic color of the surrounding skin of a festering open sore. It is the indication of the initial stages of the skin dying.
One must get rid of the open sore by washing it, soaking it, and cleaning it with hydrogen peroxide, or baking soda plus vinegar, or baking soda and fresh squeezed lime juice, all of which create a hyperbaric oxygen environment which will kill the fungi and the bacteria that is causing the festering open sore.
Blood oxygen will return to the skin when the sore is cleaned out.
It is very important to get rid of the sore as soon as possible by washing it and cleaning it everyday, several times a day. Keep the foot in open air as much as possible. This means not wearing any closed end shoes. Wear sandals. Open wounds exposed to air tend to dry up. A sore that is moist is a festering sore.