These two get used interchangeably, but there's a useful distinction. A monochrome scheme takes a single colour and works it through different tints, tones and shades — think one blue, played from palest sky to inky navy. A tonal scheme is a bit more relaxed: it layers closely related colours that share a temperature or undertone, so you might bring a soft green into a scheme that's mostly grey-blue. Same family feeling, slightly more breathing room.
Why bother? Because contrast is tiring, and a tonal room is restful without being flat. The trick is that depth comes from texture and the *steps* between your tones, not from clashing colour. You want enough difference between walls, woodwork and the next layer that the eye reads separation, but not so much that anything jumps out.
The classic mistake is picking shades too close together — the room goes muddy and the effort disappears. Leave a proper gap on the LRV scale between elements. A good rule: walls mid-tone, woodwork a clear step lighter or darker, and one anchor that's noticeably deeper.
For a green tonal scheme, Farrow & Ball's Mizzle on walls, Treron on joinery and Card Room Green as your deep anchor gives you a beautifully graded run. Prefer something softer and putty-toned? Little Greene French Grey - Pale with a warmer mid-grey works a treat. For a properly elegant near-monochrome, Farrow & Ball Cornforth White through to Mole's Breath stays in one grey family while still reading as three distinct layers.
"But won't it look boring?" Only if it's all one finish. Tonal schemes live or die by texture — matt walls, eggshell trim, a wool throw, a linen curtain. Mix the surfaces and the colour does the talking quietly.
Practical advice: pull samples and stand them next to each other in the actual room light before you commit. If two boards blur into one another, push one further. The contrast you see on a brightly lit shelf always shrinks once it's on the wall, so be braver with your steps than you think you need to be.