My dog (Chow Chow) after shedding has become a very light color. It was dark brown, but now it's almost yellow.

This is absolutely normal phenomenon, plus in winter and autumn the coat darkens, as much depends on the diet, the dullness of the coat is usually due to a lack of B vitamins.

You can try Catosal B12: https://homelabvet.com/product/catosal-10-vitamin-b12-genuine-100ml/