7.2. Do not convey information by colour alone
Do not use color as the sole differentiator of information in content. This applies to text, and images, and rich content.
For example, a chart might show different data segments using coloured lines. However, changing the lines’ thickness or making them dotted or dashed would also help convey the information in a way that doesn’t rely on the user having to discern the colours.
Tip
The first way to test this principle is to imagine the page printed in black and white, and see if all the information can still be understood. Would you be able to see and understand the information it contains?
Warning
If the images or rich content have been supplied for publication with information conveyed by color only, they should be returned to the person who provided them and a new optimized version should be obtained.