Humans are not the only ones to age, turns out, dogs age too and when they do, their fur undergoes changes just as in humans. Dogs may not get white hair to the extent of humans, but it’s normal to see some graying of the muzzle. As a dog matures, it’s quite common for his darkly pigmented coat to attain a lighter color, explains veterinarian . This loss of pigmentation in the dog’s coat occurs because pigment takes quite a good amount of energy to make and the body focuses on tending towards other more important needs. As long as there is no irritation, redness or undesirable smells, there should be nothing to worry about.
fairly large black and brown spots on Almost all the white is.
Anyway, you didn’t mention how old your niece’s Shih Tuz is. Is he a purebred or a mix? Some dogs fur (and dogs with hair also) will change color as they age. It doesn’t necessary change evenly either. It could be part of the aging process. Also would have to do with the colors of the parents, one being more dominant in color than the other. Just guessing. When we first got Katie, Maltipoo, she was apricot in coloring all over. She was bred with a Maltese (white of course) and a scarlet color toy poodle. Her head has turned almost completely white like her Maltese father, her body has just recently started getting lighter too. The texture of her body and leg hair is poodle like and remains so, not at all Maltese except for her head which is more Maltese like. If nothing else has changed in your nieces dog’s life then that would be my best guess. Just a guess though.
Plus she has been crying a lot lately
However, I'm not opposed to future folklore about me including "started the pets-as-hair-color-inspiration trend," so I enthusiastically urge you to consider your dog/cat/rabbit/hamster's fur (or iguana/snake/fish's scales!) the next time you're thinking about a change. It hasn't strengthened my relationship with Rufus (or caused a rift in my relationship with , for that matter), but I no longer have to envy that lucky dog's natural color.
My Dogs Fur Is Changing Color - Puppy & Dog Forums