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Subtractive, not additive
Print works backwards from screens. A monitor starts black and adds light; paper starts white and inks subtract wavelengths from the light reflecting off it. Cyan ink absorbs red, magenta absorbs green, yellow absorbs blue. Layering all three should theoretically produce black — in practice it produces a muddy dark brown, because real inks aren't perfect absorbers.
Why the K exists
That imperfection is one reason for the fourth channel, K (key, i.e. black ink). But black ink earns its place for practical reasons too: it's cheaper than layering three colored inks, it dries faster with less ink soaking the paper, and it prints small text far more sharply — three-ink "black" text shows colored fringes at the slightest misregistration between printing plates. This is also why print designers distinguish plain K:100 black from rich black (a mix like C:60 M:40 Y:40 K:100) — rich black looks deeper on large areas, but plain black is the only safe choice for body text.
The gamut problem
CMYK physically cannot reproduce everything a screen can show. Bright saturated oranges, vivid greens, and electric blues sit outside the CMYK gamut entirely — the press simply has no combination of those four inks that gets there. When an RGB design goes to print, out-of-gamut colors get compressed toward the nearest printable color, which is the single biggest reason "the printout looks duller than my screen." If a color must match precisely across print runs — brand colors especially — print workflows use spot-color systems rather than CMYK mixing.
Why CMYK conversions are approximate everywhere
There is no single correct RGB-to-CMYK formula. The honest answer depends on the specific press, inks, and paper, which is what ICC color profiles encode — coated paper absorbs ink differently than uncoated, and every profile converts differently. The CMYK values shown on this site's color pages use the standard device-independent approximation, which is exactly what every general-purpose converter shows: right for reasoning and rough matching, but final print work should always be proofed through the print shop's actual profile.
Practical handoff tips
When preparing web colors for print: convert early and evaluate, don't just convert at export time — an out-of-gamut brand color deserves a deliberate CMYK-side decision, not a silent compression. Keep text pure K. And if a client compares a business card against their phone screen under office lighting, know in advance that they will never quite match — one is emitted light, the other is reflected; the physics guarantees a difference before any formula is involved.