Color Accessibility

Color accessibility means designing so that meaning survives when color does not. Labels, line styles, and markers let readers distinguish a chart’s series without relying on hue alone.

For color vision specifically, MedlinePlus gives red-green deficiency as about 1 in 12 males and 1 in 200 females among people with Northern European ancestry, with lower rates in other populations studied. In a large audience, some readers are likely to have a red-green deficiency, and in a small one there may be none; because a chart cannot know which readers it will have, design so that a red line and a green line can be told apart without relying on hue.

WCAG 1.4.1 states the rule: “Color is not used as the only visual means of conveying information, indicating an action, prompting a response, or distinguishing a visual element.” The requirement is redundancy, not restraint — pair hue with a direct label, a line style, a marker shape, or a printed value, and the information survives the loss of color.

Contrast comes with numbers rather than adjectives. Criterion 1.4.3 requires at least 4.5:1 for text, relaxing to 3:1 for large text — 18 point, or 14 point bold. The figure is derived, not arbitrary: WCAG explains it “compensated for the loss in contrast sensitivity usually experienced by users with vision loss equivalent to approximately 20/40 vision”, adding that 20/40 “is commonly reported as typical visual acuity of elders at roughly age 80”.

Charts carry a second obligation that governs the drawing itself. Criterion 1.4.11 requires 3:1 contrast for “parts of graphics required to understand the content”, and the Understanding document gives each line in a graph as an example of such a graphical object. A chart whose labels all pass can still fail on its pale data series when readers must discern those lines to understand the chart, because the series color is usually chosen for looks and the text color for legibility. The criterion does not apply where labels and values shown on the chart convey the same information as the graphical objects, or where a particular presentation of the graphic is essential to the information.

References

• WCAG 1.4.1 Use of Color
• WCAG 1.4.3 Contrast (Minimum)
• WCAG 1.4.11 Non-text Contrast
• WHO, Disability
• MedlinePlus, Color vision deficiency