22 min read

Colour Modes: RGB and CMYK

Screens mix red, green and blue light; printers mix cyan, magenta, yellow and black ink. Choose the right mode for the job.

Two Ways to Make Colour

RGB vs CMYK

$RGB$	$CMYK$
$Additive — light added to black$	$Subtractive — ink on white paper absorbs light$
$Screens: websites, social media, video, presentations$	$Printing presses: posters, flyers, brochures$
$Larger range (gamut) — very bright, saturated colours$	$Smaller range — very bright blues, greens and oranges look duller$
$Values 0-255 per channel, or hex (#1FA2A6)$	$Percentages of each ink (0-100%)$

Some bright screen colours cannot be made with ink — they are out of gamut for CMYK. When a design is converted to CMYK or printed, those colours shift to the nearest printable colour, often looking duller. Designers check this before printing, not after. In Photoshop, View \to Proof Colors shows a preview of how the design will look in CMYK, and View \to Gamut Warning highlights colours that cannot be printed.

Colour mixer (RGB)

Red31 → 1FGreen162 → A2Blue166 → A6

Hex: #1FA2A6

rgb(31, 162, 166)

≈ CMYK 81% 2% 0% 35%

The CMYK figures use a simple formula. Real print conversion (for example in Photoshop) uses colour profiles, so the numbers will differ a little.

Try it:

Slide green and blue to full and red to 0 — a very bright cyan-green. Real print conversion would dull it slightly. Compare the RGB numbers with the approximate CMYK percentages.

Practice

In RGB, what do red, green and blue light at full strength make?