All 6 Uses

subtle
in
The Guardian, by Nicholas Sparks
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He noticed the subtle differences as she changed before his eyes.
Chpt 23 *

subtle = slightly noticeable
Slowly, ever so subtly, Mike's eyes began to travel their way again.†
Chpt 6
No, I was thinking of something a little more subtle.†
Chpt 10
You aren't exactly the most subtle person I've ever met.†
Chpt 22
Why be subtle?†
Chpt 22
Not that she was dumb enough to forget the possible risk—I will never be one of those abused guests on morning talk shows that everyone in the audience considered an idiot for not seeing it coming, she told herself—but a subtle change had taken place, without her particularly being aware of it.†
Chpt 29

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)