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…