RGB
RGB is the three-channel color model — Red, Green, Blue — used by every screen and most digital art tools. It can display brighter, more saturated colors than CMYK, which is why your file looks vivid on a monitor but often duller in print.
Why it matters
You design in RGB by default; you only notice the gap when you export for a printer that wants CMYK and the proof comes back flatter than expected.
Example
A neon-green RGB value (0, 255, 100) has no exact CMYK equivalent — the closest printable color is noticeably darker and less saturated. Picking colors that fall inside the CMYK range keeps screen and print closer.