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Welcome to the June 2006 edition of our monthly

Accessibility Tip

Each month we would like to bring to you a tip on how you can improve the accessibility of your website. Each tip will contain details on a particular issue that faces people with disabilities, how it affects people and what you can do to resolve it.

This month we are discussing: Colour Blindness and Colour Contrast.

The issue

About one in twelve people have some form of colour vision impairment. It affects predominantly males, around 8% of all men, but there are some cases of female colour vision impairment, around 0.4% of woman.

There are a number of different forms of colour vision impairments:

Deuteranomaly
Have a reduced perception of green in colours and have some difficulty in distinguishing certain shades of green, red and yellow.
Protanomaly
Have a reduced perception of red in colours and have some difficulty in distinguishing certain shades of green, red and yellow.
Deuteranopia
Have a severe reduction in the perception of green in colours and have difficulty in seeing shades of green against black or very dark backgrounds.
Protanopia
Have a severe reduction in the perception of red in colours and have difficulty in seeing shades of red against black or very dark backgrounds.
Tritanopia
Have a severe reduction in the perception of blue in colours and have difficulty distinguishing blue from green and yellow colours as well as blue against very dark backgrounds.
Achromacy
Have no colour perception at all and can only distinguish between the brightness of colours. This is similar to watching TV on a Black and White set.

Low contrast vision affects especially older people. This makes it difficult to read small fonts or fonts with insufficient contrast to the background.

People with these kinds of impairments may have difficultly accessing the information on your site if you use colours with insufficient contrast or if you use colour combinations that are difficult to distinguish for certain forms of colour blindness.

What you can do

You can download a free tool for MS Windows which will analyse the contrast between different colours from http://www.visionaustralia.org.au/info.aspx?page=628. It also includes a simulation of the selected colours against various forms of colour blindness.

Here is an online colour contrast analyser as well: http://juicystudio.com/services/colourcontrast.php.

When you are designing your site make sure that you use colours with sufficient contrast especially for text on background or for icons. Use the tools above to test the colour combinations to ensure that it is easy to read the text.

If possible provide the user with different options for viewing your site (skinning). In addition to your normal view you can provide a text-only view, high-contrast view etc. This will allow the visitor to select the format that best suits their visual needs.

Also ensure that the user can increase the font size on your site. This will make it easier to read the text especially for somebody with very low-contrast vision.

Bruce Aylward, 06 June 2006

About W 3 A

W 3 A provides consultancy and audit services on information accessibility, focussing on Internet accessibility, compliance with the W3C standards and NZ Government Web Guidelines, and website best practice. We have relationships with several charities and government agencies responsible for the interests of the disabled.

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Previous issues of the Accessibility Tip is available on our articles page.