CMYK ↔ RGB Color Converter (Print Color Space)
Color
CMYK is the subtractive four-channel color space used by every commercial printer — cyan, magenta and yellow inks absorb light, with a fourth K (key/black) channel added for sharper text and deeper shadows. Converting RGB to CMYK is not a single mathematical formula: the same gray can be made from equal CMY ink, from pure K ink, or any mixture. Pure CMY runs the press wetter (more dot gain and slower drying), pure K loses smoothness in midtones. This tool implements three common strategies — simple max-K, GCR (replaces neutral overlap with black aggressively), and UCR (replaces only in shadow regions) — and shows the per-channel ink separation plus total ink coverage, which press operators try to keep under ~300% to avoid bleed-through.
Arithmetic conversion only — not a calibrated soft proof. Real print workflows use ICC profiles and target press conditions. Keep total ink coverage under ~300% for coated offset, ~240% for newsprint.
How to use
- Pick a screen color via the color picker or paste a hex code (#rrggbb).
- Switch black-generation strategy: Simple keeps the maximum non-black, GCR pulls neutral overlap into the K channel, UCR only does this in shadows.
- Or flip the mode to CMYK → RGB, slide the C/M/Y/K percentages, and watch the screen-color approximation update.
- Check the total ink coverage — anything over 300% should be reduced before sending to a commercial press.
Frequently asked questions
- Why doesn't my screen color match the printed proof?
- Monitors emit light (additive RGB, wide gamut); print reflects light (subtractive CMYK, smaller gamut). Bright saturated blues, oranges and greens are often outside the printable gamut and will appear duller on paper. A real ICC-profile-based color-managed workflow is needed for accurate proofing — this tool gives a useful arithmetic approximation, not a calibrated soft proof.
- What's the difference between Simple, GCR and UCR?
- Simple max-K: K = 1 − max(R,G,B), then CMY are computed from that — leaves CMY at high values in shadows. GCR (Gray Component Replacement): replaces the equal portion of C+M+Y with K throughout the tonal range — saves ink and reduces dot gain. UCR (Under Color Removal): same idea but only applied in shadow areas, preserving smooth midtones. Newspaper presses often use heavy GCR; offset litho on coated stock often uses light UCR.
- What's total ink coverage and why does it matter?
- It's the sum of all four ink percentages — e.g. 60C 70M 80Y 40K = 250% TIC. Coated offset typically allows up to 300%, uncoated and newsprint much less (around 240%). Higher TIC means the paper can't absorb the ink fast enough — set-off, show-through, mottling, and slow drying are the consequences.
- What is 'rich black'?
- True black ink (100% K) prints as a slightly muddy dark gray on most stocks. Rich black mixes 100% K with some CMY (e.g. C60 M40 Y40 K100) to lay down a deeper, denser black for large solid areas. Don't use rich black for small text — slight misregistration between the four plates makes the edges look fuzzy.
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