A free image editor with CMYK for print work

A free editor with proper CMYK is Paint by Zaweba. Image > Mode > CMYK Colour keeps real cyan, magenta, yellow and black values for every pixel layer, at 16 bits each, and CMYK TIFFs open and save without a round trip through RGB. Around that sit ICC colour management, soft proofing with a gamut warning, separations as PNG, TIFF or PDF, and PDF output from the Print window.

Plenty of free editors can show you a CMYK preview, but few hold actual CMYK numbers in the document. That matters to printers, who want the black in your text to be pure K rather than a muddy mix of all four inks.

How CMYK works in Paint

Switch a document with Image > Mode > CMYK Colour. Each pixel layer then keeps its own C, M, Y and K values beside the RGB view. Pixels you don't touch keep exactly the values they came in with, so opening a CMYK TIFF from a designer and making a small fix doesn't quietly shift every colour.

Lab Colour mode is there too, keeping L, a and b values. Lab separates lightness from colour, which some retouchers like for sharpening or colour correcting without one affecting the other.

Setting up colour management

Open Edit > Colour Settings with Ctrl+Shift+K. Here you choose the working RGB, CMYK and grey spaces, the rendering intent, black point compensation and what to do when a file arrives with a different profile. Assign Profile and Convert to Profile handle files that need changing. Paint has built-in profiles including sRGB, Display P3 and Rec. 2020, and it can use any profile installed in Windows, which is where you'd put the CMYK profile your printer sends you. Exports embed the profile.

Ask the print company which profile they want. Using their profile is the single biggest step towards a print that matches your screen.

Checking before you send

View > Proof Setup picks the profile to simulate, Proof Colours (Ctrl+Alt+Y) shows the picture as it will print, and Gamut Warning (Ctrl+Shift+Y) turns colours the press can't reach grey. Bright blues, greens and oranges are the usual culprits. Bring them back gently with a saturation or hue adjustment until the grey patches disappear or are small enough not to matter.

You can also view individual plates, C, M, Y or K, or CMY together, which helps spot stray colour in a black and white logo or too much ink in deep shadows. A Soft Proof adjustment layer is available if you prefer to keep the preview as part of the layer stack.

Getting files to the printer

File > Export > Separations writes the C, M, Y, K and spot plates as PNG or TIFF files, or as one PDF with labels and registration marks, and optional halftone screens. That suits screen printers and anyone producing their own plates. For a normal print shop, a CMYK TIFF or a PDF from the Print window, which has Save as PDF, is usually what they ask for.

Things to watch

Convert to CMYK once, near the end, from your best RGB master. Going back and forth loses colour each time. Keep text and logos in solid colours rather than photos, so they print crisply.

Paint by Zaweba is free software under its EULA, with no subscription and no account, and it runs offline on Windows.