Not unusual at all, many poodle, terrier, schnauzer, shih tzu breeds are considered “brindle” and change colors, usually go lighter. Black turns to charcoal, brown, red tones, the whole gamut.
Unless the dog is chewing on it’s paws and legs due to allergies, that could cause discoloration also. I doubt the food has anything to do with it 🙂
This is not veterinary advice; consult your veterinarian.