"Subtle" is an adjective. It describes a small difference. "There is a difference in sound between a Baldwin piano and a Steinway piano, but it is very subtle."
"Nuance" is a noun. It means one of many slight different shades of color, taste, meaning. "His acting lacks nuances. He is either angry or sad, nothing in between." "Did you catch the nuance? He never admitted he had made a mistake, only that mistakes were made."
And, you can, of course, say "subtle nuances," and people do say it, but it's redundant because nuances are always subtle.