If you're new to print, you may never have come across the terms RGB and CMYK — yet in printing you run into the conversion between them constantly. This guide breaks down the difference between the two, how each one affects your finished print, why your screen can't be trusted for final colour, and why printers reach for spot colours when accuracy matters.

What are RGB and CMYK?

RGB stands for the three primary colours of light — R (red), G (green) and B (blue) — emitted by a light source. Each channel runs from 0 to 255. Because the colours are built up by adding light, RGB is an additive model: the more you add, the brighter it gets. Push all three channels to 255 and you get white; set all three to 0 and you get black.

CMYK is the colour space printing actually uses — the four process inks: C (cyan), M (magenta), Y (yellow) and K (black). Each value runs from 0 to 100, and, unlike RGB, colours get darker the more you add. This is a subtractive model, exactly like mixing watercolours: more pigment, deeper colour. CMYK is a reflective colour mode — you see the colour of a printed piece because sunlight or lamplight falls on it, the inks subtract the excess wavelengths, and what reflects back to your eye is the colour you see. That's why you can't read a magazine, newspaper or brochure in a pitch-black room.

Why you can't trust your monitor for final print colour

  1. The colours on your screen are made of RGB, so they will differ from a piece printed with CMYK inks.
  2. The CMYK gamut is smaller than the gamut of a typical monitor.
  3. Gang-run printing and the printing environment introduce a 10–15% colour variance, so the result already drifts from "true" CMYK.
  4. Every paper stock absorbs ink differently, which changes the finished result.
  5. Screens display light and dark values very clearly, but where the total CMYK value is below 8% or above 250%, the difference in the printed colour is slight.

So you have to design using CMYK values — otherwise the print won't come out the way you expect.

What happens if you print an RGB file directly?

Print straight from an RGB file and the finished colours come out brighter than they looked on screen. If the designer has used sensitive colours, the result can look very unnatural — or the colour can shift outright. Print in RGB, find the result disappointing, and you're reprinting from scratch. So set your file to CMYK before you start designing, or you'll lose both money and time.

RGB CMYK
On screen
Printed result

How do you check colour before printing?

If your company works to a specific colour target or standard, refer to a four-colour process swatch book — an internationally recognised CMYK reference. Its printed swatches let you see roughly how a colour will look once printed, which is far more accurate than calibrating by monitor. Bear in mind, though, that even this can't be 100% accurate.

Use spot colours to avoid big colour shifts

Spot colours are also known as special colours, or Pantone. PANTONE is the company behind the international standard colour cards; the small colour chips they define are called PANTONE colours. To use a pigment analogy: a spot colour is a ready-made pigment, whereas four-colour printing mixes every shade from just four inks.

A PANTONE colour isn't necessarily impossible to reproduce in CMYK, and it works well as a colour standard because you can print by referencing it. Every PANTONE colour has a corresponding CMYK value. But a colour mixed from CMYK will always differ somewhat from the PANTONE colour itself and can't reproduce it perfectly, as the image below shows. During press adjustment, the aim is to get the print as close to it as possible.

*To print a PANTONE colour exactly, use spot-colour printing.

Printing with us across Hong Kong and Macau and not sure whether your file is print-ready? WhatsApp our team at +852 3001 5678 (English is fine) and we'll check your colour setup before we go to press.