Additive light (RGB)
A screen emits light. With no light the pixel is black; adding red, green and blue in full gives white. Because it can emit bright, pure hues, RGB covers a large, luminous gamut — which is why neon colors exist on screen.
- Additive: light is added
- Black is the absence of light
- Wide, bright gamut
Subtractive ink (CMYK)
A printer puts ink on white paper. Each ink absorbs (subtracts) part of the light, and what reflects back is the color you see. Adding all inks tends toward a muddy dark rather than a clean black, which is why a separate black (K) ink exists.
- Subtractive: light is absorbed
- White is the bare paper
- Narrower, duller gamut
Why bright colors go out of gamut
Vivid greens, oranges and electric blues are emissive on screen. Ink can only reflect ambient light, so it cannot reach the same saturation. Those colors are called out of gamut for print and get pulled toward the nearest reproducible color.
Converting between them
A mathematical conversion (as this site's tools do) is a useful starting point, but a faithful print conversion goes through an ICC profile for the specific press and paper. Always let the printer make the final conversion for production work.