The difference between RGB and CMYK is one of the most fundamental concepts in print production, and one of the most common sources of colour surprises for clients who are new to commercial printing.
How RGB works
RGB stands for Red, Green, Blue. It is an additive colour model — colours are created by adding light. A screen starts with black (no light) and adds red, green and blue light in varying intensities to create colours. When all three channels are at maximum intensity, the result is white. RGB has a wide colour gamut — the range of colours it can represent — particularly for vivid, saturated colours.
How CMYK works
CMYK stands for Cyan, Magenta, Yellow, Key (Black). It is a subtractive colour model — colours are created by subtracting light. A printed piece starts with white paper (full reflection of light) and applies inks that absorb (subtract) certain wavelengths. The CMYK gamut is smaller than the RGB gamut. There are colours that a screen can display that simply cannot be reproduced with CMYK inks on paper — particularly vivid blues, greens and oranges.
Why this matters for print
When you design on screen in RGB and then print in CMYK, the colours must be converted. Some colours convert cleanly — neutral greys, most reds, many browns and earth tones. Others shift significantly — vivid blues often become duller and more purple; vivid greens can shift towards yellow or teal. It is always better to convert to CMYK yourself, check the result, and adjust if necessary before supplying the file.
When to use Pantone spot colours
For brand-critical colours — a logo, a brand colour that must be consistent across all materials — CMYK simulation may not be accurate enough. Pantone spot colours are pre-mixed inks that produce a specific, defined colour regardless of the press or substrate. If your brand has a defined Pantone colour, printing with that spot ink will produce more consistent results than a CMYK simulation.
Practical guidance
Always set up your design documents in CMYK from the start. If you are working with images from a camera or stock library, they will be in RGB — convert them to CMYK in Photoshop before placing them in your layout. After conversion, check the colours carefully. For brand colours, always work from defined CMYK values (from your brand guidelines) rather than converting from RGB or Pantone.