CMYK vs RGB: why printed colours look different

Light and ink work in opposite directions

A screen starts dark and adds light. Each pixel mixes red, green and blue at different strengths, and full strength of all three reads as white. Paper starts white and ink takes light away. Cyan, magenta and yellow each absorb part of the light that hits them, and black, the K, is added because the three coloured inks together make a muddy brown rather than a true black. Vistaprint's guide to the two models puts it as additive mixing for RGB and subtractive mixing for CMYK, and recommends RGB for websites, social media and digital ads and CMYK for business cards, flyers, posters and packaging. Web images and design tools usually work in sRGB, the default colour space the CSS colour specification defines for colours on the web.

Why some colours cannot be printed

Each system can produce a certain range of colours, called its gamut. A good screen reaches intense blues, electric greens and glowing oranges that four process inks on paper simply cannot match. When a file is converted for print, colours outside the ink gamut are moved to the nearest colour the press can make. Vistaprint's guide describes the result of printing RGB files without conversion: bright and neon colours lose their pop, prints can look less vibrant, and some colours may change noticeably. Darker, earthier and more muted colours usually survive almost unchanged, which is why a deep navy flyer and a hot pink flyer can print with very different accuracy from the same machine.

When the difference matters

It matters most for brand colours, large flat areas of a single colour, and designs built around vivid blues, greens, purples or neon tones. It matters less for photographs, which contain a wide spread of colours that shift together and still look natural. Uncoated paper narrows the printable range further than coated or glossy stock, because ink soaks in and spreads, so the same file looks softer on a matte uncoated flyer. Where a colour must be exact every time, such as a logo on packaging, printers use a premixed spot ink instead of building the colour from four process inks. Pantone's article on spot against process colour sets out that trade off. A spot ink adds a separate plate or pass on the press, so it usually costs more than standard four colour printing and is reserved for jobs where the colour carries the brand.

Common mistakes

The most common is choosing a brand colour on a bright screen and expecting paper to match it. The second is converting a file to CMYK with a generic profile instead of the printer's own, which can shift colours more than letting the shop convert. The third is building black text from all four inks, called rich black, for small type; slight misregistration leaves coloured fringes, so small text should be 100 percent black ink only. The fourth is judging colour on an uncalibrated laptop screen at full brightness. The last is skipping a proof on a colour critical job. A printed proof on the actual stock is the only reliable preview of how the colour will land.

Colour in the flyer builder

The flyer builder works in RGB from start to finish. PDF, PNG and JPG exports are RGB files, and there is no CMYK export, no colour profile embedded, no spot or Pantone colour option and no print proof. Colours come from the 99 themes, and text can also take a custom hex colour. A print shop receiving the file converts it to CMYK using its own press profile, which is often the most accurate conversion available because it matches that press and paper. To keep surprises small, favour theme palettes with deeper, less saturated colours for print, avoid neon accents in large areas, and ask the shop for a printed proof when colour matters. The same design exported as PNG or JPG for social media stays in RGB, where screens show it as designed.

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