Red-green color vision deficiency affects roughly 1 in 12 men and 1 in 200 women, making it one of the more common visual differences designers need to account for. The practical accessibility implication is straightforward: information should never be conveyed by color alone, a required form field marked only with red text, or a chart that relies only on red versus green to distinguish two data series, can become unreadable or ambiguous.
WCAG success criterion 1.4.1 requires an additional visual indicator, such as an icon, pattern, label, or text, alongside color whenever color is used to convey meaning.