RGB vs CMYK: when screen colors betray you in print and how to fix it
RGB (Red, Green, Blue) is an additive color model for screens: you start with black and add colored light to create colors. CMYK (Cyan, Magenta, Yellow, Key/Black) is a subtractive model for print: you start with white paper and add ink that absorbs light. The fundamental difference is why a vibrant neon green that looks amazing on your monitor prints as a muddy olive.
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How this is calculated
The RGB color space is much larger than CMYK. RGB can represent roughly 16.7 million colors (8 bits per channel). CMYK, being limited by real-world ink chemistry and paper absorption, can reproduce far fewer. This is the gamut problem. When you send an RGB file to a commercial printer, the printer's RIP software converts RGB to CMYK, and any colors outside the printable gamut get mapped to the nearest reproducible equivalent. The result is almost always duller and darker than you expected. The fix is to design in CMYK from the start when you know the output is print.
Verdict
Design in RGB for anything that lives on a screen (websites, apps, social media, presentations). Design in CMYK for anything that will be printed commercially (brochures, business cards, packaging, magazines). If you're unsure, design in RGB and convert to CMYK before sending to print, checking for gamut warnings in your design tool.