Hi Jean,
I am not sure about nutrition causing coat color changes, but I do know of a host of other things that it can be. The reddish coat on a black dog can be the result of constant contact with saliva, if it is an area that is licked by the dog herself or her housemate. Dead hair also is a reddish color and I am used to it generally on the ears and legs of intact, as well as neutered, dogs. I have a puppy, 11 months, and she is still shedding out dead puppy coat on her ears, which is a reddish color and seeing that I am showing her I try to get as much as possible out. I generally hand strip my dogs ears, they come out beautiful, thankfully I have a few friends that are terrier people and they showed me how.
Good luck with the girls, and Merry Christmas,
Jenn Tower
Aero - Flatcoat (almost 6yo)
Tuck - Golden Mixbreed (blind and best puppy nanny, almost 5yo)
Joy - Flatcoat (11 months)
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