Graphics - Damn it Gamut!


Monitors produce colours by mixing various amounts of Red, Green and Blue light (abbreviated RGB). For example, 50 per cent of red and blue will produce purple, but some other colours are not so obvious: 100 per cent red and 100 per cent green will produce yellow.

When an image is viewed on paper, the observed colours appear by a different process. If you have a red sheet of paper and shine a white light on it, the paper absorbs all the colours except red. The red light is reflected off the page and that is the colour you see. It is only the reflected colours that give an object its colour.

To make everything a bit more complicated, most printers generate images using a combination of just four inks: Cyan, Yellow, Magenta and Black (abbreviated CYMK). For example, red is printed on a page by using a mix of magenta and yellow inks.

Due to the different ways that colour is generated, there are certain colours that can only be accurately displayed on screen, while there are specific colours that can be displayed only on paper. Examples of the limitations include pure cyan and yellow - neither of these can be displayed accurately on a monitor. The range of colours that can be produced by a monitor, printer or any colour system is called a Gamut.

You can set PhotoShop to warn you when you are using colours that are outside your Gamut by selecting View-Gamut Warning or to make a manual adjustment select File-Preferences-Transparency & Gamut. This will make sure that on screen colours match the printed versions.

When working with images your monitor should be set to 24bit or 32bit colour. To adjust this setting in Windows, click Start-Settings-Control Panel-Display. Then select the Settings tab - you can change the display under the "Color palette" menu. Another area to watch is the calibration of monitors and printers - if they don't match, then what ends up in print may be entirely different to what appears on screen. Several video drivers come with calibration software while high end graphics packages also have their own tools. In PhotoShop, select Help-Adobe Color Management Wizard and then click Open Adobe Gamma. To calibrate your printer it is best to check the manual, help files or the manufacturer's Web site for details.


PC World receives many questions about working with Web colours and the confusing way that HTML defines them. For example, lime green would be generated by COLOR=#A1FF22.

After the #, each pair of digits represents R, G and B settings. In the example, the value of the red channel is A1, blue is FF, and green is 22. These are hexadecimal numbers - each pair of digits reflects a range of 0-255 when converted back to the decimal system. So, A1 equals 161 on a scale of 0 to 255, and the monitor will hence display the red value as 161 (which is about 63 per cent). Since you can have 256 choices for each of the red, green and blue settings, this will allow the six hexadecimal digits to display 16.7 million colours.

If this sounds too confusing, then many HTML and graphics programs will do the conversions for you. Click on the colour picker in PhotoShop or in Paint Shop Pro (the two rectangles at the bottom of the tool bar). Here you can convert between hexadecimal, RGB, or pick a new colour.

Some final points to remember with Web graphics and colours: GIFs can only display 256 colours - any colour outside of this narrow range will be slightly altered to fit in. This is why photographs appear grainy or patchy - subtle tones are often reduced to one large block of the same colour. If you are using GIFs and trying to match them to background colours on a Web page, you may find that due to the GIF's limited range, the colours don't quite match. In these cases either change your HTML colour settings or try converting your original graphic to a JPEG - this format can display 16.7 million colours. Also be aware that Windows, Unix and Macintosh have only 216 colours that will display the same way across all platforms (also called the Safe Colour Set). Don't get too anxious about this apparent limitation. The substantial loss in quality of using just 216 colours can far outweigh the minor inconsistencies that are seen on different platforms.

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Scott Mendham

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