Colour Modes

There are lots of different graphics formats that you can store your images in. Each of these stores information about the colours used in your image in one of a variety of different ways. The way in which this information is recorded in the image file as well as the way that it is referenced from within your graphics program is called the colour mode. Each colour mode uses a different method of storing the information about the colours used in the image and most restrict the number of colours used to some particular maximum number of colours.

The most important thing to remember about colour modes is that whenever you switch from one colour mode to another that either supports fewer colours or which tracks the colours in a different way, information about the colours used in your image is lost. For this reason you need to think carefully about which colour mode is the most appropriate to work with in your program depending on what the image is and what you intend to do with it. By selecting an appropriate colour mode when you first capture or create the image will ensure that the accuracy of the colours in the image is maintained.

So what are the colour mode choices? Let's start with the mode with the broadest colour range and work our way down to the one with the smallest range.

The two of these modes that you will probably work with most are standard RGB mode for your colour images and greyscale mode when you don't need colour. If you start working in one of these two modes then chances are that the only time that you will need to swap to another mode is when you need to store the resultant image in a file type that uses a different mode. In this instance you are probably best off to save the original image using a file format that supports the mode that you are working in and then create a copy using the alternate mode. Any future changes to the image can then be made to the original before recreating the alternate mode copy. In this way you can cut down on the colour inaccuracies that are introduced each time that you change an image from one mode to another.

Your particular graphics program may or may not refer to the different modes using the names that I have used here. Some programs may instead refer to a number of channels and a number of bits per channel. Where these programs refer to four channels they mean CMYK mode, three channels is RGB mode, and one channel is greyscale mode. If the program makes reference to a palette (and the option is not greyed out) then it is using indexed colour mode. Where a program makes reference to colour depth it means either the number of colours in the palette or the number of different colour values in each channel. Where these are expressed as a number of bits the total number of colours is two to the power of the specified number.


This article written by Stephen Chapman, Felgall Pty Ltd.

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