Think of it like this: the masstone is what a colour shouts, the undertone is what it whispers. And the whisper is what catches you out.
Masstone is the dominant colour at full strength — the thing you'd name if someone asked. Cornforth White's masstone is "grey". Setting Plaster's masstone is "soft pink". Easy.
The undertone is the secondary pigment mixed in to soften, warm or cool that masstone. It's barely visible in the tin but comes alive on a big wall under real light. Cornforth White (Farrow & Ball) reads as a neutral grey on the swatch, but in a north-facing room its mauve-pink undertone can surface and surprise people — that's why folk complain their "grey" went a bit lilac. Skimming Stone has a warm yellow-beige undertone that keeps it cosy. Ammonite leans the other way, cooler and more neutral.
Why it matters: undertones decide whether two colours sit happily together. Pair a paint with a cool blue undertone next to one with a warm yellow undertone and the join looks slightly off, even if you can't say why. This is the single biggest reason whites clash. Wevet (cool) and Slipper Satin (warm, with a yellow undertone) are both "white" but won't read as the same family.
The practical test for spotting undertone:
- Paint a big sample board — A3 minimum, two coats — never judge off the tin lid or a fan deck.
- Hold it next to a true white like a sheet of printer paper. The undertone jumps out by contrast.
- View it in your room, your light, morning and evening. North light cools everything and exaggerates blue/green/mauve undertones.
My advice: stop choosing by masstone alone. Two greys with the same LRV can behave completely differently because one's green-grey and one's purple-grey. Get the boards up, live with them a couple of days, and trust what the undertone does in your light — not what the name promises.