PDF RGB vs CMYK: What Your Printer Needs

A screen mixes light from red, green and blue. A press mixes ink: cyan, magenta, yellow and black. A PDF can describe colours in either way, in several other ways, and mix them inside one file. When a printer asks for a CMYK PDF it wants to know how your colours will turn into ink. This guide explains the vocabulary and what to check.

The colour spaces in a PDF

• DeviceRGB and DeviceCMYK. Colours defined directly for an RGB or CMYK device, with no profile. DeviceCMYK values are ink percentages.
• DeviceGray. One value from black to white. Black text is very often set this way, and it prints with black ink only, which is usually what you want.
• ICCBased. A colour space tied to an ICC profile, such as sRGB or a CMYK press profile like FOGRA39, so the numbers have a defined meaning.
• Separation. A single named ink, such as a PANTONE spot colour, with an alternate space showing how to approximate it. DeviceN combines several inks.
• Lab, CalRGB and CalGray. Device-independent spaces, less common in print files.
• Indexed and Pattern. A palette of colours for an image, or a fill made of a repeating pattern or a shading.

Images each have their own colour space, so a layout can be mostly CMYK and still contain an RGB photo.

Why printers care

RGB can show colours that CMYK ink cannot reproduce, such as bright saturated blues and greens. If RGB reaches a press workflow it is converted, and the conversion can change the look, for example a vivid blue turns duller. Converting on purpose, with the profile your printer names, lets you see and control the result. Spot colours need their own plate unless you convert them, so printers also ask which ones you use.

Output intents and PDF/X

An output intent is an entry in the PDF that names the printing condition the colours are meant for, often with an embedded profile. PDF/X, the ISO 15930 family, is a subset of PDF for print exchange: it requires an output intent, a TrimBox or ArtBox for each page and embedded fonts, and PDF/X-1a also limits the file to CMYK and spot colours without transparency, while PDF/X-4 allows transparency and ICC-based colour. A file can declare a PDF/X version in its metadata without actually conforming, so a declaration is a claim, not a result.

Checking what a PDF uses

The PDF Colour Space Checker for Print reads the page content and resources in your browser and lists the colour spaces actually used to paint, by page and by object type, with spot colour names, ICC profile descriptions, the images and their colour spaces, output intents, a declared PDF/X version, transparency features and which pages have trim and bleed boxes. A colour that is defined but never painted is not counted, and text without a colour operator is reported as DeviceGray, as it is drawn. It is a preflight inspection: it does not say a file is print-ready.

Fixing a file

1. Find the RGB objects with the checker, noting the pages.
2. Convert them in your layout application using the profile the printer gave you. Convert the whole document, not only the objects you noticed.
3. Export with the printer's settings (often a PDF/X preset), including bleed. If you only need to add bleed to an existing PDF, the PDF Bleed Adder can do that.
4. Check again and look at the result with your printer's own preflight. Fonts matter too: see the PDF Font Checker. If the job is printed in black and white, the grayscale tool shows what the shades will look like.