Both Uses of

subtle

in

To Kill a Mockingbird

I was beginning to notice a subtle change in my father these days, that came out when he talked with Aunt Alexandra.
p. 178.7

subtle = slightly noticeable

Until my father explained it to me later, I did not understand the subtlety of Tom's predicament: he would not have dared strike a white woman under any circumstances and expect to live long, so he took the first opportunity to run— a sure sign of guilt.
p. 221.8 *

subtlety = non-obvious nature

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 understandable by someone with adequate sensitivity and relevant knowledge (perhaps depending upon fine distinctions)

or:

capable of understanding things that require sensitivity and relevant knowledge (perhaps understanding fine distinctions)