Guide
RGB is for screens. CMYK is for ink. Mixing them up is the most common cause of color shifts in print. Here's how each system works and how to manage the boundary between them.
Red, Green, and Blue are additive colors they combine to make white on a backlit screen. RGB can represent a wider gamut (a broader range of vivid colors) than CMYK, which is why photographs look so vibrant on screen. The limitation is that RGB literally cannot print on paper in the same way: there is no "RGB ink".
Cyan, Magenta, Yellow, and Key (Black) are subtractive colors that work by absorbing light. They simulate the full color spectrum by overlaying tiny dots of these four inks at different sizes. The combination of all four at full coverage produces black. CMYK's gamut is narrower than RGB some neon and saturated colors that look vivid on screen simply cannot be reproduced with standard printing inks.
When you convert an RGB image to CMYK in Photoshop or Acrobat, the software maps your screen colors to the nearest printable equivalents. The result depends on your color profile not just "CMYK". A vivid orange that works in RGB might come out muted in CMYK without the right profile. This is not a bug; it's the nature of the gamuts. The fix is to use the correct color profile for your print vendor.
Converting to CMYK can push coverage higher than expected especially in shadow areas and dark photographs. Use a coverage analyzer to check each page after conversion. If any page exceeds your print vendor's limit (usually 300–340%), adjust the color values or pull some ink from heavy areas before submitting.
Check ink coverage