RGB vs CMYK: when to use each
How the additive RGB model and the subtractive CMYK model differ, and why a color can look different on screen and in print.
Additive vs subtractive
RGB is additive: screens emit red, green and blue light that add up to white. CMYK is subtractive: ink absorbs light, and cyan, magenta, yellow and black combine on white paper to make darker colors. That is why the brightest RGB colors cannot always be reproduced in CMYK.
Why the numbers shift
Converting RGB to CMYK with the naive formula is deterministic, but real printing uses ICC color profiles that map colors to a specific press and paper. The CMYK values here are a reliable starting point for screen mockups, not a substitute for a print proof.
Picking a workflow
Design for screen in HEX, RGB or HSL. When a file is bound for print, convert to CMYK early and check the result with your printer's profile so nothing looks washed out on paper.