WCAG Contrast Check

Contrast ratio is decided by lightness alone. Lose the colour difference and the ratio doesn't move. Red and green clear 4.5:1 whenever their lightness differs enough. They still look like one colour to someone with deuteranopia. A ratio checker cannot see this failure, because the number it reports doesn't change. ■ The vision tab (free) Shows the pair as it appears under protanopia, deuteranopia and tritanopia, with the contrast ratio after simulation and **how much of the colour difference survives**. Red (#D32F2F) on green (#388E3C) keeps 13% of it under deuteranopia. That pair is only 1.21:1 — which is the wrong test for it. Nobody sets body text in red on green; the two get used as status colours, where all that matters is whether they can be told apart. The simulation uses the matrices from Machado, Oliveira & Fernandes (2009), applied **after converting to linear RGB**. Applied to gamma-encoded sRGB instead — the easy mistake — pure red comes out #5E4700 rather than #A39000. The colour difference is the distance across the a*b* plane in CIELAB and excludes lightness. Only the colour is lost, so measuring with lightness included returns a comfortable answer that isn't true. ■ Try one (free) Pairs that actually break are ready to tap. Black on white has no colour to lose, so nothing would happen and you would never see the point. • Error red and success green — the usual status pair. 13% under deuteranopia • Passes AA, still hard to tell apart — 6.72:1, and 19% under tritanopia • A pair that doesn't rely on colour — nothing is lost ■ Contrast ratio (free) Judged against the five WCAG 2.2 criteria: 4.5:1 for normal text, 3:1 for large text, 7:1 and 4.5:1 for AAA, 3:1 for UI components. Large text means 18pt (24px) or larger, or 14pt (18.66px) or larger in bold — written on the screen rather than hidden behind a toggle. ■ Pro (one purchase, not a subscription) Works out a foreground colour that meets the threshold, by binary search, so it moves the colour the least amount that passes rather than jumping to black or white. Saving palettes is Pro too. Everything above stays free. ■ Built to be usable Works offline. No account. No tracking. Nine languages.