CMYK vs RGB

Red, Green, and Blue (RGB) are the three primary colours of light. In any light generating device (like computer screens), RGB blends these three colours to produce all other colours.

There are two types of colour profiles: additive and subtractive. RGB is an additive profile because you create new colours by adding primary colours of light together.

Green light + blue light = cyan Blue light + red light = magenta Red light + green light = yellow Red light + green light + blue light = white

With RGB, the colour gamut is very wide, so it can produce a broad range of colours. This is because it essentially mirrors how our eyes see colour. In our retinas, we have three different kinds of cone cells that pick up on red, green and blue light. Our brain then blends together the different signals that the cells receive, producing an astonishing array of colour.

Of course, this all depends on one key factor: light. Anything that’s printed doesn’t generate light, it reflects it. So, how do you reproduce colour in print, if there’s no light source?

The answer is that you don’t add, you subtract, which is where CMYK comes into play.

The RGB colour model

CMYK stands for Cyan, Magenta, Yellow and blacK (also known as K for Key). These four colours of ink are blended to create colours on items that reflect light, like paper or fabric.

Unlike the RGB, where you create colours by adding two or more together, CMYK is a subtractive colour profile, so you you remove a primary colour of light from any white light that hits it.

White light (such as daylight) appears visible to the naked eye, but that doesn’t mean it’s void of colour. In fact, white light occurs when all colours of the visible spectrum appear in roughly equal amounts. By playing with these amounts, we can subtract wavelengths and produce different colours.

• Cyan ink reflects green and blue light, but not red
• Magenta ink reflects blue and red light, but not green
• Yellow ink reflects red and green light, but not blue

To produce the three primary colours, you have to think in reverse.

• By combining cyan and magenta, you only reflect blue light
• By combining magenta and yellow, you only reflect red light
• By combining yellow and cyan, you only reflect green light

Imagine you have a piece of white paper. It’s white because it reflects every wavelength of light. By using cyan, magenta and yellow in different proportions, you subtract specific colours of light to produce a wide gamut of colours.

Now, in theory, by combining cyan, magenta and yellow you should get black. But our eyes perceive colour the RGB way, meaning the CMY doesn’t produce a perfect result. To combat this, we add in black ink (key). The black ink will absorb all light and provide darker tones and richer blacks.

The CMYK colour model

Everything printed on paper will have a CMYK ink colour profile. Similarly, anything created to be viewed on a computer screen will have an RGB colour profile. So if you’re creating a booklet for example, you will want to start your designs in CMYK and print in CMYK.

However, if you were to create a webcomic, you would begin by designing in RGB so it can be better viewed on the web. Then you would convert your files to CMYK and manually adjust any colours that may not look quite right before printing them. This is because starting in CMYK for online projects can restrict the creative process due to the more limited choice of colours.

To get a quick impression of what your RGB project will look like in CMYK, simply switch to CMYK Mode in Photoshop or InDesign. Then you can easily see which colours may need adjusting.

If you are using a colour profile we recommend GraCOL2006.

If getting the perfect colour is crucial to your project, we recommend ordering a proof of your project to see how it turns out. If you have any questions, please contact our team of print experts. We’re always happy to help.