The phrase "subtle undertones" refers to a hidden or faint message or quality that is not immediately obvious, but can be sensed or felt. Full definition
Ask your stylist to give you a side part with Hollywood-esque curls and brighten up your chocolate brown shade with subtle undertones of violet. (prettydesigns.com)
And the wines were both unique and festive with the traditional flavors of a sweet white or wine, but with subtle undertones of the featured flavors of Apple & Caramel and Mixed Berries & Chocolate — the flavors were unmistakably present, but not overpowering. (cupcakesandkalechips.com)
I used 1/2 cup of honey in lieu of the sugar and it adds a delicious subtle undertone. (epicurious.com)