Undertone
The slight lean of a colour towards another hue, seen best when it is thinned or compared.
Also: bias, cast
Few colours are perfectly neutral. A white may lean yellow or blue; a red may lean orange or purple. That lean is the undertone. It shows most clearly when two similar colours are held side by side, or when paint is spread thinly. Undertones matter in practice. Two greys that look the same on small cards can clash on a wall if one is warm and one cool. In make-up and clothing, skin is often described as having a warm, cool or neutral undertone, a rough guide to which shades will suit.