RGB and CMYK describe colour in different physical worlds. RGB uses red, green and blue light from a display; CMYK uses cyan, magenta, yellow and black inks on paper. A design can look luminous in RGB and still reproduce perfectly well in print—but not always with the same numbers or the same saturation.
The aim is not to force every file into CMYK at the beginning. It is to prepare it for the actual output condition and make colour changes intentionally.
RGB is light; CMYK is ink
RGB is additive: more light moves toward white. CMYK is subtractive: more ink generally absorbs more light and moves toward a darker result. The white of a printed page is the paper itself, so stock colour, coating and viewing light all affect the perceived image.
Why colours change in conversion
Many bright greens, blues, oranges and fluorescent-looking screen colours cannot be matched by process CMYK ink. When they are converted, an ICC profile maps them to the closest useful output values according to a rendering intent. The result depends on the chosen profile, paper, press and ink limit.
That is why there is no single “CMYK version” of an RGB colour. A CMYK build for coated offset stock can be unsuitable for uncoated paper, a digital press or newsprint.
Should you deliver RGB or CMYK?
Follow the printer’s specification. Many modern PDF workflows accept profiled RGB imagery and convert it at the RIP using the print condition chosen for the job. Others request a CMYK PDF, supplied CMYK images or a named PDF/X standard. Both approaches can be correct when controlled properly.
| If the printer asks for… | A reliable approach |
|---|---|
| Profiled RGB / PDF/X workflow | Keep images in RGB, embed profiles and export with the requested PDF preset. |
| CMYK artwork | Convert a copy using the specified CMYK ICC profile; retain the RGB master. |
| Brand-critical colour | Discuss a proof and whether a spot colour is appropriate before conversion. |
| Mixed ink job | Keep process and spot swatches named correctly; check separations before release. |
Process colour and spot colour are different tools
CMYK builds an image from four process inks. A spot colour is a separately specified ink, often used for a brand colour, a limited-palette design or a special printing effect. A spot colour can sometimes reach a hue that CMYK cannot, but it still changes with paper and finish.
Do not assume a Pantone name is a guaranteed visual result in every situation. See Pantone and PMS explained and confirm the intended book, substrate and printing method with the supplier.
Make conversion predictable
- Work from profiled source files and keep an editable RGB master.
- Ask for the print condition before choosing a CMYK profile or export preset.
- Soft proof important imagery and brand colours for that condition.
- Convert once, deliberately, if the workflow requires CMYK.
- Check the final PDF in output preview and obtain a proof for critical colour.