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Colour contrast is important for people who have vision impairments, including people who are colour blind. One in five men has some form of colour blindness, so this is a common problem.
There are different types of colour blindness caused by defective rods and cones in the eye, however the three most common types of colour blindness are:
People without colour blindness see:
Depending on the type, people with colour blindness see:
Checkpoint 2.2: Ensure that foreground and background color combinations provide sufficient contrast when viewed by someone having color deficits or when viewed on a black and white screen
The W3C developed an algorithm to ensure adequate colour contrast . This algorithm uses the hexadecimal values of foreground and background colours to determine if the colour contrast is sufficient. The W3C algorithm is not yet a recommendation and some experts have said that it is too strict.
Instead of using the algorithm every time you need to test your colours, you can use the JuicyStudio Colour Contrast Analyser, which automatically calculates the algorithm for you.
The Colour Contrast Analyser provides two outputs:
Colour brightness refers to the difference in brightness between the foreground colour and the background colour. Colour brightness must be a value of 125 or higher. Colour difference refers to the difference between the foreground and background colours. Colour difference must be a value of 500 or higher.
There are two ways to test a current site for adequate colour contrast:
You can use Juicy Studio if you know the hexadecimal values of the colours. (You can use the Iconico ColourPic tool if you don’t know the hexadecimal values).
You can use Vischeck (or one of the other colour blindness simulators), however you need to be aware that this does not utilise the W3C algorithm. You can test an entire web page with Vischeck , however style sheets etc are not well-supported. Alternatively you can test an image with Vischeck . If you have Adobe Photoshop or ImageJ you can download a Vischeck plugin which applies a filter to images. The easiest way to test a page for colour contrast is to take a screenshot of the page (by pressing the “Print Screen” or “prt sc” button on your keyboard) and upload it via the image facility or filter it using one of the plugins.
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