RGB vs CMYK: additive vs subtractive, screens vs print

Designers run into RGB vs CMYK the first time they send a web mockup to print and the colours look wrong. The two systems aren’t alternative ways to label the same colour; they describe colour in fundamentally different physical mediums. Light vs ink, addition vs subtraction, wide gamut vs narrow.

The fundamental difference

RGB is additive — light combines

Your screen has red, green, and blue subpixels. They start dark and add light to produce colours. All three at full intensity make white; all three off make black. Intermediate combinations make everything in between.

CMYK is subtractive — ink absorbs

Printed paper starts white (reflecting all wavelengths) and is darkened with cyan, magenta, yellow, and key (black) ink. Cyan ink absorbs red, magenta absorbs green, yellow absorbs blue. Combining ink reduces what the paper reflects. All four at 100% should theoretically make black; in practice, they make a muddy dark brown, which is why the K (key, i.e. black) ink exists.

The gamut problem

RGB displays — even modest sRGB ones — produce vivid saturated colours like neon blue, electric pink, fluorescent green. CMYK ink mixes simply can’t reach those colours. The CMYK gamut is meaningfully smaller, especially in the green-cyan and orange-red regions.

When you convert RGB to CMYK (any software does this; we don’t because the conversion requires an ICC profile for the specific paper + ink combination), out-of-gamut RGB colours get clipped to the nearest representable CMYK colour. The bright orange you see on screen becomes a duller orange in print. The vivid teal becomes a muted teal.

When to use each

RGB wins when

• Output is a screen. Web, mobile, video, presentation. Screens speak RGB natively.
• You want the widest gamut. RGB’s broader colour reach is the right choice when display is the destination.
• You don’t know the final medium yet. Design in RGB; convert to CMYK as a last step if print is one of the destinations.

CMYK wins when

• Output is print, definitively. Magazines, books, packaging, business cards.
• You need print-accurate previews while designing. Designing in CMYK from the start, and using soft-proofing in your design app, lets you see what the printed result will actually look like.
• Spot colours matter. Pantone and brand-specific spot inks live in CMYK’s adjacent world. Working in CMYK keeps you aware of their constraints.

The conversion path

Convertitive doesn’t convert RGB to CMYK because the conversion is genuinely lossy in a way that depends on the target print conditions. To go from RGB to print you need:

1. The source colour space (typically sRGB).
2. The target ICC profile — typically “US Web Coated SWOP” or “ISO Coated v2” for general commercial print.
3. A rendering intent — perceptual (smooth gradients, slight overall shift) or relative colorimetric (out-of-gamut colours clipped, in-gamut preserved).

Adobe Photoshop, Affinity Designer, and InDesign all do this correctly when you specify the target profile. Most online “RGB to CMYK converters” ignore the profile question and produce one specific (often wrong) mapping. If the conversion matters — for an actual print job — use a proper colour-managed application, not a free web tool.

The pragmatic advice

• Design in RGBif you don’t know the destination. RGB is the wider gamut; you can always subtract on the way to print.
• Convert to CMYK once, at the end.Don’t flip back and forth. Each conversion loses fidelity.
• Get a printed proof before the press run. Even with calibrated software, paper warmth and ink colours vary between presses. A proof on the actual paper stock at the actual press settings is the only way to know for sure.
• Test brand colours on the target medium. Pantone gives you exact specifications for ink-based print, but the screen rendering of a Pantone-numbered colour is an approximation. Specify both your RGB equivalent and the Pantone number for any brand asset going to print.