You pick a gorgeous vivid blue on screen, send the file to a printer, and what comes back is noticeably duller. Nothing went wrong — you just ran into the fundamental difference between how screens and printers make colour. Understanding it turns “why did my colour change?” into a predictable, manageable part of design.
Convert between the two models as you read with the RGB → CMYK and HEX → CMYK converters.
Light vs. ink: opposite directions
The two models work in physically opposite ways:
- RGB is additive. A screen starts black and adds red, green and blue light. More light = brighter. All three at full = white. (Details in RGB & HEX Explained.)
- CMYK is subtractive. Print starts with white paper and adds ink, which absorbs (subtracts) light. More ink = darker. The inks are cyan, magenta and yellow — the opposites of red, green and blue — plus key (black).
Because one builds up from black with light and the other builds down from white with pigment, a colour made of light and the “same” colour made of ink are never a perfect match.
Why CMYK needs a separate black
In theory, full cyan + magenta + yellow should make black. In practice it makes a muddy dark brown, uses three times the ink, and drenches the paper. So printing adds a dedicated black plate (K) for true darks, crisp text and efficiency. That is why it is CMYK, not just CMY.
Gamut: the range each can show
The key concept is gamut — the set of colours a system can actually reproduce. A screen’s RGB gamut is larger than typical print’s CMYK gamut. Some bright, saturated colours a monitor emits simply cannot be mixed from ink:
| Colours that shift most in print | Why |
|---|---|
| Vivid blues & purples | Beyond what cyan + magenta ink can reach |
| Bright greens | Screen greens are especially out of CMYK gamut |
| Neon / saturated oranges | Limited by yellow + magenta density |
| Pure RGB primaries | No single ink equivalent; always approximated |
When a colour is “out of gamut,” conversion clamps it to the nearest printable colour — which is exactly the dulling you see. Muted, mid-tone and pastel colours usually print close to what you designed; the vivid ones are where surprises live.
#000000) can convert to “rich black” using all four inks, which on small text causes registration blur. For body text, use 100% K only. Conversely, designing large solid blacks with K alone can look weak — there rich black helps.Designing for print without surprises
- Know your destination early. If it is going to paper, keep CMYK in mind from the start rather than converting at the very end.
- Soft-proof the vivid colours. Convert your most saturated RGB colours to CMYK and back, or use a CMYK preview, to see which ones shift — then choose colours that hold up.
- Use 100% K for small black text; reserve rich black for large solid areas.
- Ask your printer for their profile. CMYK is not one fixed thing — it depends on the press, ink and paper. A specific ICC profile gives the most accurate conversion.
A note on transparency
CMYK is about which inks land on opaque paper, so the on-screen idea of an alpha (transparency) channel does not exist in a flat CMYK print the way it does in an RGBA image — transparency is resolved during layout, not stored as a fifth ink. If you are wrangling transparency in your source images before they go to print, see Colour Depth, Channels & Alpha.
In practice
Work on screen in RGB, but treat CMYK as the reality check for anything headed to print. The RGB → CMYK converter gives you the ink breakdown instantly so you can spot out-of-gamut colours before the print shop does. For the full picture of how all the colour models relate, see Color Spaces Explained.
Frequently asked questions
Why does my colour look different when printed?
Screens create colour by emitting red, green and blue light (additive); printers create it by layering cyan, magenta, yellow and black ink that absorbs light (subtractive). The two methods can reproduce different ranges of colour, so vivid screen colours that ink cannot match shift to the nearest printable colour.
What is colour gamut?
A gamut is the full range of colours a system can reproduce. The RGB gamut of a screen is larger than the CMYK gamut of typical printing, especially for bright, saturated greens, blues and oranges — those are the colours most likely to change when you print.
Should I design in RGB or CMYK?
Design on screen in RGB, but if the final output is print, keep an eye on CMYK from the start and soft-proof your most saturated colours. Converting RGB to CMYK only at the very end can cause unwelcome surprises on exactly those vivid colours.
What does the K in CMYK stand for?
K is the "key" plate — black ink. Black gets its own channel because mixing cyan, magenta and yellow only makes a muddy dark brown, wastes ink, and soaks the paper. A dedicated black gives crisp text and true darks.