Collocations for subtle - adj.

be, seem
become

extremely, fairly, very, etc.
extraordinarily, incredibly
infinitely
infinitely subtle tonal gradations

not exactly, not too
He was not exactly subtle about it!

Subtle is used with these nouns:
allusion, alteration, aroma, aspect, balance, blend, change, clue, colour, detail, difference, discrimination, distinction, effect, flavour, fragrance, gesture, hint, humour, influence, lighting, message, nuance, perfume, point, racism, reference, reminder, shade, shading, shift, sign, stripe, suggestion, tactic, touch, transformation, undertone, variation, way