What is CMYK, and why does my colour change?

The two systems work in opposite directions. A screen emits light and adds colours together — full red, green and blue light makes white. Ink absorbs light and subtracts from it — the more ink, the darker the result. They can represent overlapping but different sets of colours, and RGB’s set is larger.

The colours that suffer most are the ones a screen renders as pure emitted light: vivid oranges, bright greens, electric blues and anything fluorescent. On screen they glow. In CMYK they land somewhere duller and slightly shifted, because there is no ink combination that reaches them. Nothing has gone wrong; the colour was never printable to begin with.

The fix is to design in CMYK from the start, so what you approve is what can actually be produced. Where a brand colour genuinely has to be exact — a corporate blue, a recognisable pack colour — the answer is a spot ink rather than a process build, because a spot colour is mixed to that colour rather than simulated from four.

Substrate changes the outcome too. The same CMYK values print differently on coated board, uncoated stock and kraft, because the surface decides how much ink sits on top and how much sinks in. If colour matters, judge it on a printed proof on your actual substrate and not on any screen.