Simone Rossi

My framing is that colour decisions on a page are made against a background, and most contrast tools get used against white when the background is not white. Contrast is defined on relative luminance, and the formula has a gamma correction in the middle that is easy to omit and produces wrong answers at every step. The result is a ratio, and the accessibility thresholds are defined on that ratio. I show the calculation rather than a number appearing from nowhere. Large text and small text have different thresholds, and the distinction is based on the rendered size and weight rather than on the nominal font size. That is worth stating because a heading at the same nominal size as body text passes for a reason the author did not choose. Colour models each solve a different problem. RGB with gamma correction matches a screen. HSL is convenient for choosing hue and lightness but is not perceptually even, which is why equally light colours differ so much in contrast. HSLuv exists to fix that and is worth knowing about. Gradients need a contrast check at both ends and in the middle, because a gradient that passes at its extremes can fail where the two ends meet. Palette generation is a starting point rather than an answer. I cover what a generated palette guarantees about spacing between hues and what it does not, which is perceptual difference. Finally, colour is not an accessibility topic on its own. A pair of colours can pass every ratio and still be indistinguishable to a reader with a colour vision deficiency, which is a different test with a different method, and passing one says nothing about the other.