Colour is an important component of any visual design. We can use colour to highlight an important point we want to draw attention to in our content. We can use colour in graphs to show different categories of information. Or we can use colour in a table to highlight particular cells of content.
In all these cases we are using colour to convey information. Yet the use of colour alone to convey information can make this content inaccessible to people who have difficulties distinguishing colour. This includes people with vision impairment who often have limited colour vision, older users with age-related impairments and those who are colour blind.
The purpose of Success Criterion 1.4.1: Use of colour is to ensure that information conveyed via colour is also conveyed in another way that is accessible to all users:
Colour is not used as the only visual means of conveying information, indicating an action, prompting a response, or distinguishing a visual element. (Level A)