Understanding Colour Modes for Medical Illustrations

Frequent questions when creating medical illustrations include; what is colour mode? What is CMYK? And my image doesn't show up properly in PowerPoint / Online / when I print it. By understanding colour mode you can avoid these questions.

Photoshop image mode selector

What is colour mode?

Colour mode is simply the way that a computer holds and processes the data used to create a colour image. The reason why there are two colour modes is because one is for printing and one is for screen display and the difference in how you make colour in these situations.

Colour for Screens

Computer and TV screens make your eye perceive colour by shining light towards you. Your eyes have receptors to see red, green and blue light; white light being all three in equal amounts. The computer screen creates an image on its screen by changing pixel by pixel what colour light shines from that point. All of the colours visible to the human eye can be made up from varying the amount of Red, Green and Blue light transmitted from a point. Therefore a computer needs to hold the information for an image in a format that tells it how much Red, Green and Blue light it has to show at each point of the image. This is the RGB colour mode. As all computers have screens you would think that this is the only colour mode that they would need, unfortunately not.

Colour for Printing

When your eye looks at a printed picture the light isn't being shone out of the paper but instead the light is being reflected off the paper. A printed image is made up of ink who's job it is to reflect and absorb different wavelengths of light so we see the image correctly. The printing process uses four inks in its process to be able to reflect any colour you want from the paper. These four inks are Cyan, Magenta, Yellow and Black, which gives the CMYK colour mode. This means that if the computer is going to send an image to the printer it has to convert the data that it uses to display the image on the screen (RGB) to a CMYK format.

Why does it matter?

Well the first thing to say is that most of the time it doesn't matter. Every day we print colour documents from our computers without even thinking about colour mode. Computers handle changing from screen view to print very easily so users don't have to worry about it. However, when you come on to high quality images, such as medical illustrations, and brand logos the quality of the colour reproduction becomes important. When you leave the computer to do all the colour conversion automatically you can end up with colour variation. This can mean that colours can end up muted or possibly wrong. The more times an image file has its colour mode changed the greater the evolution of the colour from what the artist intended.

How do prevent problems with colour mode?

Its simple to prevent problems with colour mode by telling the artist what the image is going to be used for, printing or online. If a single image is going to be used for both, the artist can create both CMYK and RGB versions and then be happy that the colours are right for both before sending them through.

So that's it then?

Well not completely, when it comes to printing the actual type of paper you print on can affect the colours. So this is why there are different CMYK colour profiles to match different standard papers.
And finally you might see Pantone colours mentioned - this is again related to printing. When big blocks of the same colour is required it is better to use a paint of that specific colour rather than always build it up from Cyan, Magenta, Yellow and Black, as it is more consistent. So the Pantone colour reference is the paint reference if you like. Again these can vary by paperstock... but I think you get the picture!

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