Your design looks beautiful on screen, but in print the colours disappoint. That is down to the difference between CMYK vs RGB: two ways of building colour. Your screen works with RGB, the printing press with CMYK, and not every screen colour can be printed one to one. Understanding that prevents disappointment when supplying print files.
RGB: colour for screens
RGB (red, green, blue) is the colour space of screens. Combining light creates very bright, luminous colours. Everything you see on a monitor or phone is RGB.
CMYK: colour for print
Printing uses CMYK (cyan, magenta, yellow and black). The press mixes these four inks on paper. CMYK cannot reproduce every bright RGB colour, which is why vivid blues and greens come across slightly more muted in print.
How do you prevent colour differences?
- Work and supply in CMYK, not in RGB.
- Use a standard print profile (for example ISO Coated / FOGRA).
- When in doubt, request a proof for a reliable colour impression.
- Avoid bright RGB colours if exact reproduction matters.
Is colour crucial for your book? Request a physical proof and you will see in advance exactly what the end result looks like.
The role of colour profiles
An ICC profile describes exactly how colours are translated from one space to another. For printing in Europe the FOGRA profile is the standard. If you supply with the right profile, the printer knows precisely how your colours are meant.
You can read more about colour management and ICC profiles at the International Color Consortium, the organisation behind the standard.
A colour profile tells the press how the numbers in your file translate into real colour. Without a profile, 100% cyan is nothing more than a number; with a profile the press knows which blue should come out of it on your paper. So save your images with the profile embedded and nobody has to guess along the way.
Coated paper calls for a different profile than uncoated paper, because uncoated paper absorbs more ink and the colour therefore turns out duller. If you do not know which profile to choose, just ask before you convert everything; undoing it afterwards costs more work than setting it up correctly beforehand.
Colours you will never match exactly
Some colours simply do not exist in ink. Bright signal colours, neon green, electric blue and saturated orange look wonderful on your screen, but fall back to the nearest printable colour when converted to CMYK. That is not a fault, that is the limit of four inks on paper.
So convert your images to CMYK yourself and see what happens; then your own book will not surprise you. If an exact colour is crucial, for instance a brand colour on the cover, a spot colour in offset is the only way to really achieve it. For everything below that: a proof gives you certainty that no screen can offer.
Frequently asked questions about CMYK vs RGB
Do I always have to supply in CMYK?
For print, yes. If you supply in RGB, the printer converts it to CMYK and you can get unpleasant colour differences that you were unable to check yourself.
Why has my blue turned purple in print?
Bright blue sits close to the limit of what CMYK can handle and therefore shifts quickly towards purple. A spot colour or a proof prevents this.
What is a proof?
A physically printed sample on which you see the real colours before the whole run is printed. Recommended when colour matters.
Should I convert to CMYK myself?
That is preferable. If you supply in RGB we convert it, but then the default conversion determines the result and not you. Converting yourself shows you in advance what happens to your bright colours.
Why does my black-and-white photo look tinted?
Because a grey tone is built up from four inks and a small excess of cyan or magenta already gives a cast. Convert black-and-white photos to greyscale or to a neutral CMYK build.