If your parakeet’s foot was simply slightly more purple than the other foot, and it was not affecting his behavior, I would suggest this problem was a pigment-related issue. However, the fact he can not use the foot to stand on means the coloring is probably due to a decrease in circulation. As the circulation becomes worse or the longer it continues, the more damage will be done to the local area. He may have full use of the limb but no feeling (much like "pins and needles" in humans), and therefore not stand on it, or he may be losing the functionality of the limb. Loss of circulation creates a lack of oxygen supply to the cells in the area, which will eventually lead to necrosis and loss of the limb. You will need to have your bird examined by a veterinarian as soon as possible.