Cmyk Proof

CMYK Proof

Screen design tools speak sRGB hex. Presses speak CMYK ink under a specific ICC profile. Saturated screen colors (neon reds, electric blues, vivid greens) silently desaturate at print time, and a naive hex-to-CMYK formula conversion tells you nothing about it. This skill closes the gap: every color is proofed through a real press profile before it touches the canvas, so what the client approves on screen is what the press can actually make.

Requirements

• ImageMagick 7 (magick on PATH).
• A press ICC profile. Free and redistributable: PSOcoated_v3 or ISOcoated_v2 from eci.org (European standard). GRACoL2013_CRPC6 ships with Adobe apps (US standard, common digital-press target). Ask the print vendor which profile they proof against; when in doubt, coated defaults above are right for most commercial work.
• The script auto-detects installed profiles, or takes --profile <path> / the CMYK_PROFILE env var.

Quick start

# brand hex -> press recipe + the hex to actually use on canvas
node scripts/cmyk_proof.mjs hex '#FF003D' '#2F6F95'
# #FF003D -> C0 M96.2 Y74 K0 -> canvas #E72038 (out of gamut, drift 32)
# #2F6F95 -> C79.2 M38.5 Y13.5 K19.1 -> canvas #2E6F95 (safe, drift 1)

# recipe (percent) -> canvas hex, for colors born in ink
node scripts/cmyk_proof.mjs cmyk '0,96,74,0'

# tints of a recipe, without minting new colors
node scripts/cmyk_proof.mjs tint '20,70,0,15' 15 20 40

# soft-proof a whole exported board
node scripts/cmyk_proof.mjs image export.png proof.png

Verdicts: safe (drift ≤ 8/255 per channel), shifts (≤ 24), out of gamut (> 24). Every row also reports TAC (total area coverage, the four ink percentages summed); the script warns above 300%, a common coated-stock limit. Confirm the vendor's actual limit. --json for machine-readable output, --intent and --tac to override defaults.

The born-CMYK workflow

The law: only print-simulated hexes enter the canvas, and production files are rebuilt from recipes, never converted from canvas hexes.

1. Every candidate color goes through the script before use. Canvas gets the canvas hex from the output, not the original.
2. Record the recipe with the color wherever the tool allows: token descriptions, style notes, a palette board. Each color is dual-value, recipe for the press, sim hex for the screen. When print files are built (InDesign, Affinity, or direct to the vendor), colors are re-entered from the recorded recipes.
3. If a brand color is out of gamut, do not quietly use it. Show the owner the original next to the print simulation, offer in-gamut candidates, and get an explicit pick. If the unprintable original must stay in the file as a reference, keep it raw and visibly labeled not printable, never as a reusable token.
4. Tints come from the tint mode (scaled ink recipes), not from screen opacity. A 20% opacity overlay and a 20% ink tint are different colors.
5. Judge candidates in context: proof swatches sitting on the background color they will print over, never isolated on white.

Ink discipline

• Body text and fine rules: single-ink black (cmyk '0,0,0,100', or a K-only tint for grays). Four-ink builds on small type risk registration fringing and color casts.
• Large dark solids: rich black, e.g. cmyk '60,40,40,100' (confirm the vendor's preferred build). A hex near-black converted from screen becomes an accidental four-ink build, the TAC warning will usually catch it.
• Grays: K-only tints via tint '0,0,0,100' <pct>, not CMY mixes.
• Avoid reversing small type out of multi-ink builds for the same registration reason.

Canvas hygiene

• If the tool supports variables or tokens, bind every swatch and fill to them. After copying nodes between files, verify the bindings survived, copies often flatten to raw hex. Re-check by reading values back, not by eye.
• The canvas is the spec, not the print artwork. Exports are for approval and mockups; final artwork is rebuilt in a print-capable tool from the recorded recipes and the geometry spec.
• Before client review of near-final boards, run the full export through image mode and look at the proof, not the raw export.

Physical geometry

Pixel-native tools also lack print units, bleed, and fold handling. See references/print-geometry.md for the ppi math, bleed/trim/safe setup, and fold-allowance rules.