Contrast Ratio
Contrast ratio describes the calculated difference in luminance between two colours. At level AA, WCAG requires at least 4.5:1 for text and 3:1 for large text (success criterion 1.4.3), and likewise 3:1 for meaningful graphics and user interface components (success criterion 1.4.11).
In practice
Free contrast checkers show quickly whether a colour combination meets the requirement. Sufficient contrast helps not only people with a visual impairment but everyone reading in poor light or on a reflective screen.