It is important to consider how you use colour in your online content (web pages, PDFs, or other online material) to ensure that people with colour blindness, colour vision deficiencies, partial sight or low vision, can access all the information in that content.
Accessibility standard requirements
The New Zealand Government Web Accessibility Standard requires that web pages conform to the Web Content Accessibility Guidelines (WCAG) 2.0.
Success criterion 1.4.1 requires that colour “is not used as the only visual means of conveying information, indicating an action, prompting a response, or distinguishing a visual element.”
If colour is the only way that specific information is provided, then this information won’t be available to users who are unable to see or easily differentiate between colours.
Examples of conveying information by colour alone include:
displaying links in a different colour, but not providing any other visual cues, such as an underline
indicating errors simply by highlighting them in red
showing required form fields in another colour without any other indication, such as an asterisk
distinguishing elements in a graph with only differently coloured lines.
One of the most common ways to address this issue is to ensure, where information is conveyed by colour differences, that same information is also available in text or through another visible indicator, such as an underline, pattern or shape.
Success criterion 1.4.3 requires that the “visual presentation of text and images of text has a contrast ratio of at least 4.5:1.”
The aim of this requirement is to ensure that there is enough difference in the lightness, or relative luminance, of text and background so that the text is readable by people with low vision and those who have difficulty seeing colours.
Colours can be defined (in the HSL colour model) using 3 attributes: hue, saturation, and lightness (or luminance). While changes in hue and saturation tend not to affect the legibility of text for people with colour vision deficiencies, changes in lightness can. For this reason, a good luminance contrast ratio is essential for assisting with the readability of content for people with relevant vision impairments. If that includes a large portion of your target audience, then a higher luminance contrast ratio of 1 to 7 is recommended.