Why your colours onscreen don't look the same when printed

Have you ever wondered why your images onscreen don't come out looking the same when printed? Below I discuss:

• how the CMYK model works
• why RGB files don't look like CMYK files
• how to convert an RGB file to a CMYK file for printing purposes

The CMYK Model

The CMYK colour model is the one used for the colour printing process and it's why we all buy inks coloured cyan, magenta, yellow and black.  That's because CMYK stands for cyan, magenta, yellow and key black.  (It's that last one which always had me puzzled!)

CMYK Model of how colours combine

The colour printing process is subtractive.  Which basically means that light is subtracted to achieve the colour before it meets our eyes.

• Cyan is the complement of red and can be thought of as minus-red, 
• Magenta is the complement of green and can be thought of as minus-green, and 
• Yellow is the complement of blue and can be thought of as minus-blue.
• Black is what you get when you remove all colour by overlapping all the colour inks  (i.e., no light reflected, fully absorbed by ink)

CMYK Separation:
Cyan (C),
Magenta (M),
Yellow (Y),
Black (K),
Cyan + Magenta (CM),
Cyan+Magenta+Yellow (CMY),
CMYK

However when mixed they actually form what looks like a near black - which is why we also need a black ink to get the proper tone.

Creating a colour print involves layering the different inks to create colour and creating separate plates for any litho printing process.

The graphic above indicates six simple colous and black.  However varying both the percentage of each colour used and the ratio of one colour to the next creates a huge number of colours which span the spectrum of possible hues in terms of tone and intensity

Try taking a look at this TruHue Demo created by Dick Nelson to understand how adjustments of the quantity of cyan, magenta and yellow vary the colours they create.

Why RGB files don't look like CMYK files

You may not realise that:

• When we create an image with a camera or a scanner, we create one using technology which uses the RGB colour model (an additive model - where light is added to create the colour)
• However if you want an image printed the printer needs a CMYK colour model created using at least four colour inks which depend on chemical pigments to achieve the desired hues.

It's one of the reasons why what we look at one the screen does not print out quite as we expect.  Colours can look off or lose their intensity.

Another reason is that the RGB model actually has a higher number of hues than the CMYK model.

How to convert an RGB file to a CMYK file

You can avoid a lot of problems if you convert your RGB file ( suitable for your computer) into a CMYK file (suitable for printing out)

• This is an online CMYK converter - why not have a go?
• How to Convert RGB to CMYK in Photoshop is an article which provides a lot of detail about how to go about this conversion in Photoshop
• Color conversion (RGB / CMYK / HSV / YUV / ...) is an online page which helps you convert an RGB file to an CMYK file

However if a CMYK colour file is converted back into an RGB file information is lost.

Just like language translation, color conversion is not always a straight-forward one-to-one mapping. Specifically, RGB has less degrees of freedom (3) than CMYK (4). Therefore, although one can convert any RGB to CMYK without losing information, the inverse is not the case. When a CMYK color is converted to RGB, some information is lost. When you convert that RGB color back to CMYK, you will in most cases get a different (but similar) color.
Color conversion (RGB / CMYK / HSV / YUV / ...)

Link: 

• Colour science, systems and models - Resources for Artists
• The Best Art Books - Colour
• What is colour?
• What is colour?  A scientific perspective
• A Matrix of Colour Space Theories