Designing for print media brings with it unique considerations. Digital media looks quite similar no matter which display it is on (subject to screen calibration); a picture will look nearly the same on a phone screen or on a computer monitor for example. When submitting graphics to be printed however, the colours onscreen will not necessarily transfer to the final printed page.
The issue comes from the fact that there are multiple modes of colour. We see a full spectrum of colours in light but technology is restricted to displaying smaller ranges of colour. Additionally, the image on a screen is made from emitted light rather than reflected light like a printed image. The colours of a picture that has been printed out are affected by ambient lighting and can look different depending on the light they are illuminated by. A transmissive display does not suffer from this problem.
RBG: The Additive System
Digital technology, such as computer monitors, mobile phone screens, digital cameras and scanners use the 'additive' RGB system. This means that the colours on the display are created by combining different quantities of red, green and blue light. For example, to show a yellow image, the display produces green and red light so that the human eye perceives it as yellow.
Up close, a digital display is made from thousands of pixels, each emitting a different colour of light to form the images onscreen. RGB is the default colour mode for many design applications because it offers the widest possible selection of colours; using different levels of the three base colours allows for screens to show any colour on the conventional colour wheel. In the additive RGB system, combining red, green and blue will create a plain white light.
CMYK: The Subtractive System
The majority of image editing programs use the RGB system but, when going to print, the technology used to create printed media uses the CMYK system instead, so-called because its four colours are Cyan, Magenta, Yellow and 'Key' for black.
CMYK is a 'subtractive' system; the more colours that are added, the darker the result will be. New colours are produced by subtracting colours from each other. Subtracting yellow and cyan, for example, will create the colour green. Combining cyan, magenta an yellow, however, would only create a dark brown colour. With the additive system, adding more light colours can only make the resulting colour lighter, but with opaque inks the result is darker. This is why the CMYK separately subtracts the key colour - black - to control the shade.
The Printing Challenge
The challenge stems from the fact that media is designed on screen and is displayed in RGB mode but the printer uses the CMYK system instead. The RGB system offers a greater range of colours than CMYK and is capable of depicting colours which are particularly vivid or saturated. These colours are outside of CMYK's colour range – or 'colour gamut' - and cannot be accurately replicated, typically resulting in an image which is dull or less vibrant than the one on the screen when it was designed. This can pose an issue when media has been carefully composed but the printed copy does not match it.
So what's the solution? One option is to convert the images from RGB to CMYK before sending them to be printed. Although monitors still display colours in RGB format, it is still possible to get an impression of what the printed result will look like. RGB-to-CMYK translation software converts all of the colours of an image to their nearest CMYK-compatible equivalent to approximate an image which is as close to the original as possible while still optimal for printing.
Software packages like InDesign permit the user to adapt their work to the alternative colour system retrospectively. Most sophisticated graphics software can be configured to work in CMYK from the start. In Adobe Photoshop, for example, select: Menu -> Image -> Mode and choose CMYK when beginning a new project.
Understanding the difference between the additive and subtractive systems and the challenge they pose for printing media is important in creating designs which will be consistent between digital and physical mediums and ensuring the resulting print is to a high standard.