An undertone is the subtle bias hiding underneath the main colour — the green, pink, blue or yellow that only shows itself once the paint's on the wall and the light's doing its thing. It's why a "grey" can read lilac at dusk or a "white" can go faintly lemon in a south-facing room.
The trick is comparison. A colour on its own tells you almost nothing — your eye has no reference. Put it next to a purer, cleaner sample and the undertone jumps out. Lay a chip of true neutral grey beside Farrow & Ball Cornforth White and you'll see its mauve-pink lean immediately. Do the same with Pavilion Gray and the green-blue bias shows. This is the single most useful habit in colour theory: never judge a colour in isolation.
Three things drive how strongly the undertone reads:
- Light direction. North-facing rooms cool everything and pull out blue and grey undertones — that's why warm-undertoned colours like F&B Setting Plaster behave so much better there. South light warms everything and softens cool tones.
- What's next to it. A pink undertone beside a green sofa will sing; beside terracotta it vanishes.
- Time of day. Cool morning light and warm evening light can swing the same wall noticeably.
The other shortcut: read the colour's family. Most off-whites declare their hand. F&B Slipper Satin sits warm and creamy; Wevet stays clean and cool; Strong White has a grey-cool backbone. Little Greene and Mylands list undertone cues in their literature too — worth reading rather than guessing.
Here's the practical bit. Never trust a printed card or a screen — they flatten undertones completely. Order a peel-and-stick sample (Lick and COAT both do these) or paint two coats on A2 lining paper, move it round the room over a full day, and watch what the light does to it. The undertone you see on the wall at 4pm is the one you're living with — not the one on the tin lid.