It's a proportioning rule, and an old interior design standby for good reason — it stops a room going one of two ways: a wall-to-wall monotone snooze, or a jumble of competing colours with no anchor.
The split works like this. 60% is your dominant colour — walls, and often the largest piece of furniture or the floor. This is the backdrop, so it wants to be the calmest of the three. 30% is your secondary — sofas, curtains, an armchair, or a feature wall in a deeper tone of the same family. It should contrast enough with the 60% to give the eye somewhere to land. 10% is the accent — cushions, lamps, art, a painted door. This is where you can be bold, because there's so little of it that a punchy colour reads as a treat rather than a shout.
A worked example: walls in Farrow & Ball Cornforth White as the 60% — a soft warm grey that sits back beautifully. Then a 30% in something with more depth, say Little Greene French Grey - Dark on a sofa or curtains, or carried onto woodwork. For the 10%, go bold — Farrow & Ball Inchyra Blue on a small side table, or accessories pulling in that deep blue-green.
The "but what about" question I always get: *does it have to be three colours?* No. The cleverest schemes use one colour family across all three slots, varying only the depth — a tonal scheme. Mylands Marble Arch No.82 as your 60%, a deeper stone for the 30%, charcoal accents for the 10%. Reads expensive, never busy.
Treat it as a starting framework, not a law. The percentages are eyeballed, not measured with a tape. The real value is forcing you to decide which colour leads and which one's just a flourish. Get that hierarchy right and the room sorts itself out.