HTML and colour contrast with content

By: Steve Wilkinson
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Introduction to colour contrast

This guide provides useful information on colour contrast and how to avoid issues with content.

Why is it important?

We need to make sure that the colour contrast between any active controls (user interface components) or meaningful graphics, are discernible from the background they are on by people with moderately low vision.

We also need to make sure that colour contrast between text and its background is discernible by people who may have moderately low vision.

We must ensure that for people with color deficiencies, the colour, hue and saturation will have an adequate contrast between text and background.
Levels of contrast for text
The contrast ratios should be treated as threshold values. Computed values should not be rounded up, for example 2.999:1 does not meet the 3:1 threshold.

Normal regular text is approximately 16px. Large text is 14 point (18.66px) and bold or larger, or 18 point (24px) or larger.

The exceptions are logotypes where there is a brand name or text within a logo, and incidental text or images of text in a user interface component that is not currently active. These exceptions do not have a minimum contrast ratio.

Rules

  1. If it is functional, it must be contrast compliant. The planned visibility must pass contrast

    • Colour contrast applies to purpose indicators, state indicators, text and anything else that is essential to understanding interactions
    • If primary functions are visible to any degree, it should be visible for everyone unless it is decoration

  2. Text always needs contrast

    • Text contrast is 4.5:1 unless very big, everything else is 3:1

  3. Do not rely on colour only to convey meaning, use secondary sources of information

    • Do not use colour as the only indication of state within a component. Colour must never be the main indicator for anything that is functional
    • When avoiding having to rely on colour for communicating information, width and height changes, secondary text-based indicators and icons are best used

Particular cases

Forms

Do not identify errors in form fields by using red to highlight them, identify the error using text. A screen reader user will benefit if the text gets added to the form field’s label element because it will get read out when the focus is set to that field.

Link colour

If links are not underlined, or not provided with another non-colour visual indication for their default state, then the colour of the link must have:

Links must be obvious and visible, otherwise they do not become distinct from the surrounding text. A common method for non-underlined links is to underline the link when in hover state, but this does not work on a mobile device.

I suggest using underlined links but if you do not use an underline, use a different visual cue to colour, or increase the link’s font weight.

High contrast versions

Making a website meet colour contrast requirements in high contrast can be very difficult, but it you can comply by having alternative versions of the website or individual webpage.

This is best achieved by having a control such as a link that switches to high contrast mode.

If this method is to be acceptable:

Tools

Offline tools

Online tools

Mobile tools

Wrapping up

Hopefully this guide has helped you to understand a bit more of using colour contrast in your content.

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