A subtle warm or cool bias within a colour that otherwise reads as fairly neutral — a 'warm white' vs a 'cool white', for example.
Colourwise estimates undertone from a colour's OKLCH hue angle: hues in the red-orange-yellow arc read as warm, the green-blue-purple arc reads as cool, with narrow transition bands treated as neutral.
Undertone is not the same as hue family — two colours can both be classified 'grey' (a neutral hue family) while having noticeably different warm or cool undertones.
Two greige paints can look almost identical in a tin but read distinctly warmer or cooler once on a large wall, because of undertone.
Undertone is one of the most common causes of a paint colour 'not working' with existing flooring, furniture or fittings.
For a paint chip or a screen colour, undertone is the hue angle of a low-chroma colour: the direction in which a* and b*, or OKLab's a and b, point away from zero. A spectrophotometer reading of two whites will show it as a small difference in b* (yellow against blue) or a* (red against green). By eye, the test is comparison: put the sample against a sheet of plain white paper and against a second neutral, and the lean of each becomes obvious where it was invisible alone.
| Colour | HSV saturation | OKLCH hue | OKLCH chroma | Undertone |
|---|---|---|---|---|
| Greige #b2a79d | 12% | 65° | 0.019 | Warm |
| Mushroom #bdb0a0 | 15% | 73° | 0.027 | Warm |
| Storm Grey #71797e | 10% | 235° | 0.012 | Cool |
| Pewter #96a8a1 | 11% | 170° | 0.023 | Cool |
| Taupe Grey #8b8589 | 4% | 338° | 0.009 | Neutral band |
Greige and Storm grey are both greys with an OKLCH chroma under 0.02, yet their hue angles are 65° and 235°: one leans towards orange, the other towards blue. Taupe grey, at 4% HSV saturation, is too weak for Colourwise to assign a lean at all. On a card these differences are faint; across four walls they are the first thing noticed.
How this was calculated: Colourwise reads undertone from the OKLCH hue angle once HSV saturation reaches 8%: 20° to 95° is warm, 130° to 320° is cool, and the gaps between are left unassigned.
A cool grey is painted beside warm oak flooring or cream woodwork.
Each neutral pushes the other further in its own direction, so the grey reads blue and the cream reads yellow. Compare candidates against the fixed element in the room, not against white card, and choose the one whose lean agrees with it.
Two whites from different ranges are used on walls and trim.
A white that looks clean alone looks dirty or pink next to a white with the opposite lean. Hold the two together before buying either.