I agree with Lori - what a terrible thing for your little dog to have to go through. I am so glad that he is doing much better now.
It is extremely unlikely that this is gangrene. Gangrene is caused by a bacteria that will only grow in the absence of oxygen and causes tissue to die quickly, so if gangrene was present your dog would be much sicker and the skin would be black and probably moist and draining. It is likely that what you are seeing is just hyperpigmentation, which is one of the response that occurs when skin has chronic irritation. Because of the extent of your dog's wounds, it is likely that this is just more of healing process.
Another possibility here would be that because your dog has been on quite a few different antibiotics, he could have developed a yeast infection of the skin. Yeast infections can occur when the normal bacterial organisms have been suppressed by the antibiotics. This is not serious and can easily be treated.
A third possibility is that your dog has a hormonal imbalance that is affecting the skin. The two most common ones are hypothyroidism (low thyroid function) and Cushing's disease (decreased adrenal gland function). Both of these can cause the changes you are describing, but these are also very treatable and nothing to get too worried about.
As long as your dog continues to eat and drink normally and all you are seeing is the color change, don't get too worried. Have him checked by your vet when they get back into town, but what you are describing is probably just more of the healing process.
Please let me know if you have any additional questions.
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