Color Accessibility Tools

Color accessibility comes down to one question asked repeatedly: can someone who is not you, on hardware that is not yours, in lighting you have never seen, read this? Designers are the worst available judges of their own interfaces, because they already know what the text says. The measurements exist to replace that broken instrument.

WCAG 2.2 answers it with a contrast ratio computed from relative luminance — a weighted sum of the linearised channels where green carries 72% and blue only 7%. The thresholds are 4.5:1 for body text, 3:1 for large text and for non-text elements that convey boundary or state. They are floors, not goals: text at exactly 4.5:1 is legible in good conditions and tiring across a long article.

The ratio's blind spots are real. It ignores font weight almost entirely, and it treats light-on-dark and dark-on-light as equivalent when the eye does not. APCA, drafted for WCAG 3, models both and reports a signed lightness contrast instead of a symmetric ratio. Run both: WCAG is what an audit will cite, APCA is what will explain why a technically-passing pair still looks thin.

Separately from contrast, roughly one in twelve men has some form of color vision deficiency — usually a shifted rather than absent cone response, collapsing part of the color space onto itself. The practical rule that follows is narrow: hue must never be the only thing carrying meaning. Lightness survives every form of CVD intact.