Colour Contrast

The measurable difference in luminance or colour between two adjacent elements, used to determine readability and visual distinguishability. WCAG defines minimum contrast ratios (4.5:1 for normal text, 3:1 for large text at AA level) based on relative luminance calculations using a standard observer model. While high contrast benefits both low-vision and CVD users, the standard contrast calculation does not account for the altered luminous efficiency functions of people with CVD, potentially leaving gaps in accessibility for colour combinations involving reds.