Should you use RGB or CMYK for printing?

Use whatever your printer asks for. Some printers convert RGB files themselves; others, such as Amazon KDP for paperback covers, ask for CMYK. Adobe recommends CMYK mode for images printed with process colours and RGB for web and email. If your printer says nothing, ask before you convert.

Key points

• RGB is how screens show colour. CMYK is how a printing press or printer builds colour from ink.
• The printer's instruction decides, not a general rule. The same RGB file can be fine at one printer and a problem at another.
• Convert at the end. Adobe says that if you start with an RGB image, it is best to edit first in RGB and convert to CMYK at the end of editing.
• A single PDF can contain both modes at once, so the file itself is the thing to check.

What RGB and CMYK are

RGB describes colour as amounts of red, green and blue light. Adobe notes that the RGB model is used by computer monitors to display colours, and that RGB is the usual choice for images that stay on the web or in email.

CMYK describes colour as amounts of cyan, magenta, yellow and black ink. Adobe's guidance is to use CMYK mode when preparing an image to be printed using process colours. It also warns that although CMYK is a standard colour model, the exact range of colours it represents can vary depending on the press and printing conditions. That is one reason to ask the printer before you convert.

What printers and platforms say

Read the scope column. KDP's colour rules are written for covers, and the KDP help page does not say that every interior image must be CMYK. If your job is a book interior, check KDP's interior guidance rather than assuming the cover rule applies.

When RGB is acceptable and when it is not

Treat the printer's answer as the input and the verdict follows from it:

• The printer accepts RGB. RGB is not a problem, but colours can shift when they are converted, so look at a proof.
• The printer asks for CMYK only. RGB in the file is something to fix before you send it.
• You do not know. Treat RGB as something to check with the printer, not as an error.
• The job is black ink only. Any colour content is worth a second look, because every colour will print as a shade of grey.
• Spot colours are used. They print as a separate ink only if you ordered it. KDP, for one, recommends converting them to CMYK.

How to convert

1. Ask the printer whether they want CMYK, which profile to use if any, and whether the profile should be embedded. KDP, for example, advises against including colour profiles in the file.
2. Finish editing in RGB, then convert to CMYK as the last step, as Adobe advises.
3. Export the PDF and check the file you will actually send.
4. Look at a proof. A conversion can change colours; our checker's report names bright blues and greens as the place a shift is most likely to show.

What we tested

We built one small test PDF with two rectangles: one filled with RGB red and one with CMYK red. We ran the same file through the RGB / CMYK Checker three times and changed only the answer to "what does your printer accept?".

In all three runs the checker found the same facts: one RGB colour setting and one CMYK colour setting on page 1, no images, no ICC profile. The file did not change; the verdict did. That is the practical answer to the question in the title.

Limits

The checker reads which colour spaces a PDF uses. It does not measure colours, predict how a conversion will look, or check total ink coverage, rich black or overprint. A result of Passed means no problem was found in the checks listed, not that your printer will accept the file. Our test file is a generated document, not a customer's artwork.

What to do next

Ask your printer which colour mode they want, then check the PDF you plan to send. Colour is one of several things a printer looks at. For page size and trimming, see How much bleed do you need for printing?. For fonts, see PDF fonts not embedded: what it means and how to fix it. For how the checks work and what they leave out, read how Check Before Submit works.