About + method
Colour, with the working shown.
RiotPad is a small, self-running tool for anyone choosing colours for something real: a shop sign, a website, a slide deck, a flyer. It does the arithmetic that tells you whether a palette will work, and shows how each number is made.
How the numbers are made
- Contrast
- WCAG 2.2 relative luminance of each colour (sRGB, linearised), then (lighter + 0.05) / (darker + 0.05). Marks: 4.5:1 (AA) and 7:1 (AAA) for text, 3:1 for large text and for buttons, icons and borders (Success Criteria 1.4.3, 1.4.6, 1.4.11). Ratios are cut to two decimals, never rounded up.
- Harmony
- The seed is converted to OKLCH (Bjorn Ottosson's OKLab, 2020) and its hue is turned by the rule's angles; lightness and chroma stay the seed's. A colour that will not fit on screen keeps its lightness and hue and loses chroma until it does.
- Ramps
- Eleven OKLCH lightness steps (50 = 97.5% to 950 = 22%), hue held, chroma as the seed's and reduced only where the screen cannot show it.
- Colour vision
- Machado, Oliveira and Fernandes (2009), "A Physiologically-based Model for Simulation of Color Vision Deficiency", severity 1.0 matrices for protanopia, deuteranopia and tritanopia, applied to linear RGB. No colour vision = relative luminance as grey. Pairs are compared by OKLab distance; RiotPad's cut-offs (0.05 "look the same", 0.10 "hard to tell apart") are a rule of thumb, not a standard.
- Names
- The nearest of the 148 CSS named colours, by OKLab distance.
Plain limits
- Screens differ, and printed colour differs from screen colour. RiotPad works in screen colour (sRGB); it does not match paint, ink or fabric colours. RiotPad names no paint, ink or fabric brand and makes no colour-matching claim.
- Contrast is one part of accessibility, not all of it. RiotPad checks colour pairs against WCAG 2.2; it does not certify a website as accessible.
- The colour-vision views are an approximation of how a typical person with that kind of colour blindness sees (Machado, Oliveira and Fernandes, 2009). They are not a diagnosis or an eye test.
Who runs it
RiotPad is built and run by WeldonPC (weldonpc.com), an independent computer services business. It has no staff reading palettes and no tracking cookies. The home page, the tools, Pricing and this page carry one labelled banner from WeldonPC’s own ad network, AdEngine (see Privacy); your Paint Box never shows one. "Riot" here means a riot of colour; RiotPad has nothing to do with games, music or any company with "Riot" in its name.