Parakeet---Foot problem?

One of my parakeet's feet has began to turn a purple color. It seems to be getting darker and he is unable to support himself with it less and less. He was trying to sleep and kept falling off his food dish so i removed perches and placed in a cloth to try to get him to sleep on that so he will be balanced. This foot is causing him to have trouble flying and he is staying puffed up. I am not sure what caused this in his foot. I have tried giving him food that is to help with illness and placing cream on his foot. Any suggestions? I am not sure what is wrong with him or if this is the proper way to handle it. Thank you.