All 3 Uses

subtle
in
The Water is Wide
(Edited)

...what has remained is an indefinable nonspeech, flavored subtly with a nonaccent, and decipherable to no one, black or white, on the American continent.
p. 38.9 *

subtly = not obvious, but noticeable
His name struck fear in the souls of all his employees and his hand ruled the passages of his carpeted bailiwick with the subtlety of a sword.
p. 189.6

subtlety = something that is indirect or non-obvious -- often harmful or intentionally hidden
It looked as though the Old South was still alive and well, a little more subtle, without the sheets and night riders, but a force that still tolerated little deviation from the norm.
p. 277.9 *

subtle = working in an indirect or hidden way

Definitions:

(1) (subtle as in: a subtle poison)
working in an indirect or hidden way
(2) (subtle as in: a subtle shade of blue)
understated so as not to draw excess attention
(3) (subtle as in: a subtle difference or thinker)
not obvious, but noticeable with enough insight and knowledge

or:

able to notice or understand things that require insight and sensitivity
(4) (meaning too rare to warrant focus)