Try an apple cider vinegar rinse after shampoo/conditioner. That may help remove any bacteria/ dandruff, etc., in case something is in the fur and causing bacteria to spread. If not, then it may be that the new hair coming in is actually a light tan. As mine age, the one who had a little brown spot on her back now has a bit larger spot and more on the ears. Her brother does not have any brown on him, but has a TON of pigmentation on his skin (poor thing looks like a little cow with the black/brown spots), when he is bathed. good luck!