There is genetic evidence of grizzlies acquiring polar genes over the eons likely due to glacial advances and retreats altering habitats and cutting-off populations temporarily.

https://en.wikipedia.org/wiki/Grizzly–polar_bear_hybrid#Anci...

This seems like the hard bit:

You can't go by the color of the fur. Black bears can be black, brown, cinnamon, blond, blue-gray or white, according to Bear.org.

One of the best ways to tell the difference, Pratt said, is to look for a hump at the shoulders. Grizzlies have them. Black bears don't.