You have designed something on screen, sent it to a printer, and got back a duller, darker version of what you were looking at. Nothing was printed incorrectly. The problem started the moment you chose the colour mode.
Two ways of describing the same colour
RGB describes colour as emitted light: red, green and blue added together. Your screen starts black and adds light, so a pixel that is fully lit in all three channels is white. CMYK describes colour as reflected light: cyan, magenta, yellow and black inks printed onto paper. The paper starts white and the inks subtract light, so adding all four inks gives something close to black.
These are not two encodings of the same information. They are two different physical processes, and some colours exist in one and not the other.
Why your screen shows colours ink cannot reach
The RGB gamut is larger than the CMYK gamut, and the gap is not evenly distributed. It is worst in exactly the colours designers reach for:
- Vivid saturated blues and purples. Pure
#0000FFis simply not printable; the closest ink mix is noticeably duller. - Bright saturated greens and oranges. The fluorescent-looking greens achievable on an OLED display have no ink equivalent.
- Highly saturated reds. Large areas of vivid red tend to print with a brownish or orange cast.
Conversely, CMYK can reproduce some deep, rich blacks and subtle earth tones that are awkward on a backlit display. The mismatch runs both ways — it is not simply that print is worse, it is that the two gamuts do not coincide.
What actually happens when you convert
Converting RGB to CMYK is not a lossless relabelling. Out-of-gamut colours must be pulled to the nearest ink-reachable colour, and the algorithm that does this is called a rendering intent. The three you will encounter:
| Intent | Behaviour | Use it for |
|---|---|---|
| Perceptual | Compresses the whole gamut to fit, so relative relationships between colours are preserved but every colour shifts slightly | Photographs with many out-of-gamut colours |
| Relative colorimetric | Leaves in-gamut colours untouched and clips only the ones that are out of range; white point is remapped | Designs with brand colours that must not shift |
| Absolute colorimetric | No white point remapping, so paper tint shows through | Proofing — simulating one printer on another |
This is why a logo can come back a different shade of blue after being converted with a different intent than last time. If a colour is critical, check which intent your export is using.
Total ink coverage, and why black matters
Printing all four inks at 100% in the same place does not produce a rich black. It produces a wet sheet: the paper is oversaturated, drying is slow, colours can smear onto the next sheet, and the result looks muddy rather than deep. Printers therefore specify a total ink limit, commonly 300% for coated stock and 240–260% for uncoated.
That is what the black channel is for. Rather than building dark tones out of equal parts cyan, magenta and yellow, a proper build uses black for the density and support inks for the hue. Two builds worth knowing:
- Rich black (for large solid areas): around
C40 M30 Y30 K100. The support colours add depth that K alone cannot reach. - Plain K100 (for body text): black text should be 100% K only. Using a rich black for small text causes misregistration — if the plates shift slightly, the text gets coloured fringes.
A practical checklist before you export
- Convert to CMYK yourself and look at the result, rather than letting the printer convert it and hoping. Anything that shifts badly is better discovered on your screen.
- Check total ink coverage against the printer’s limit, and reduce the build where it exceeds it.
- Set body text to K100 only.
- Ask the printer for their ICC profile and convert to that rather than to a generic CMYK. Coated and uncoated stock need different profiles, and the difference is visible.
- Soft-proof before you commit, so the screen simulates the ink gamut instead of showing you colours the press cannot produce.
- Do not convert back. CMYK to RGB to CMYK round-trips lose colour every time. Keep a master RGB file and convert once, at the end, for the specific output.
Converting between colour models
The PressBay colour tools convert between RGB, HEX, HSL and CMYK, and report the total ink coverage of a CMYK build so you can see whether it will exceed a printer’s limit before you send the file. Everything runs in your browser — useful when the artwork is not yet public.