CMYK

A screen adds light. It starts at black and piles red, green and blue on top of one another until it arrives at white. Paper does the opposite: it starts white, and every ink laid on it removes part of the light the sheet would otherwise have bounced back, so the only direction available is darker. Cyan ink absorbs red, magenta absorbs green, yellow absorbs blue, and two of them overprinted remove two bands at once, which is why cyan over magenta reads as a deep violet rather than as anything in between the two. The fourth ink is black, called key because its plate carries the line work, the type and the shadow detail the other three are registered against. In theory the three colours already reach black together. In practice real inks are impure, so their overlap lands on a heavy near black, uses three times the ink, and takes far longer to dry than one pass of black would.

CMYK is a model rather than a colour space. The four numbers say how much of each ink to lay down, and they only name a colour once a press, a paper and a set of inks have been specified, which is the job of a profile such as SWOP or FOGRA. Two shops printing the same 100 0 0 0 will not hand back the same cyan, and the same file on uncoated stock comes back duller than on coated, because uncoated paper drinks ink and spreads the dot. This is also the line between CMYK and halftone: halftone is how a press makes a tint at all, by growing and shrinking dots on a lattice, while CMYK is which inks those dots are made of. Four halftone screens, one per ink, each rotated to its own angle so the lattices do not beat against one another.

Four inks reach a good deal less than a display does. Saturated cyans, oranges and greens that an sRGB monitor shows without effort have no ink mix behind them at all, and the gamut is narrower again on cheap stock. That little warning triangle in a colour picker is usually saying exactly this: the value you chose is outside the destination profile and something will be substituted for it. Choosing the substitute deliberately is gamut mapping, and it is worth doing by hand, because the swap is otherwise silent and a brand colour that arrives two steps duller was nobody’s decision.

For interface work the practical rules are short. Pick brand colours against CMYK first if anything will ever be printed, since a colour born on a screen may have no press equivalent while one born on press always has a screen equivalent. Expect the printed version to read darker and less saturated than the proof on your monitor. Expect small text to be set in the key ink alone, because four plates in imperfect register turn a hairline into a coloured fringe. And do not carry ink numbers into a stylesheet: CSS has no colour of this kind to offer beyond a device-cmyk() function still sitting in a draft, so the values have to be converted to sRGB by somebody, and doing that conversion knowingly is the difference between a palette that survives the trip and one that arrives muddy.