Has Yukon been spening a lot of time outside? In summer sunlight? If so, then hair can bleach in sunlight. It could turn black hair a sort of brown/gingery colour in places.
When your vet says he's fine...did she run any blood tests? And/or urine tests? Sometimes it is very difficult to tell if something is wrong, if a dog is still eating well, playing, etc, without going a little eeper, and checking bloodwork to see if there are any imbalances.
My suggestion would be to get the vet to run blood tests.
Thyroid problems, and hormonal problems as well as other health issues, can cause a dog's coat to change colour too.