Color vision deficiency
Difficulty telling certain colors apart, most often red and green, which is common and mostly inherited.
Color vision deficiency, often called color blindness, means a person sees some colors as similar or the same. Red and green confusion is the most common form. People with it see perfectly well; what they cannot do is rely on color as the only clue.
That is the design rule: never use color alone to carry meaning. A required field marked only in red, a chart with a red and a green line and no labels, or a status shown only as a colored dot leaves them guessing. Add a label, an icon, a pattern, or text. WCAG covers this under its perceivable principle.
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