When you’re working in Photoshop, it’s important to understand that there are different colour models and colour modes available. I’m going to to briefly explain the theory behind models in Photoshop and why you would choose one particular model over the other.

What is a colour model?
A colour model is simply a way to define colour. A model describes how colour will appear on the computer screen or on paper. Three of the most popular colour models are:

  • CMYK (Cyan, Magenta, Yellow, Black)
  • RGB (Red, Green, Blue)
  • Lab Colour

Let’s take a look at each of these in turn.

1. The CMYK model is used for print work and it describes colours based on their percentage of Cyan, Magenta, Yellow and Black. These four colours are used by commercial printers and bureaus and you may also find that your home printer uses these colours too. These four colours are needed to reproduce full colour artwork in magazines, books and brochures. By combining Cyan, Magenta, Yellow and Black on paper in varying percentages, the illusion of lots of colours is created.

2. The RGB model is used when working with colours destined for TV screens or computer monitors. A value between 0 and 255 is assigned to each of the colours – Red, Green and Blue. So for example if you wanted to create a purely blue colour, Red would have a value of 0, Green would have a value of 0 and Blue would have a value of 255 (pure blue). To create black, Red, Green and Blue would each have a value of 0 and to create white, each would have a value of 255.

In this situation, when we talk of “value” of colour, we’re referring to the strength of the colours in relation to each other.

3. The Lab colour model is a slightly more complex beast. It is made up of three components – the lightness component (L) ranging from 0 to 100, the “a” component comes from the green-red axis in the Adobe Color Picker, and the “b” component which comes from the blue-yellow axis in the Adobe Color Picker. Both “a” and “b” can range from +127 to –128.

When Photoshop is converting from one model to another, it uses Lab as the intermediate colour model.

So, after all that which model should you use?
If you know that your work is being sent to a commercial printer, then it’s a good idea to start your document in CMYK mode. Otherwise it’s safe to say that you can work in RGB for almost any other project. Even if you’re printing at home on your own inkjet printer then RGB is the one to go for. For any screen-based work such as websites or web graphics or DVD’s, you should always work in RGB. Your monitor works in displays in RGB so in terms of colour, what you see is what you get. If you do need to convert from one colour model to the other, it’s just a matter of choosing Image > Mode and then picking the one you need.

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