Check any text color against any background for WCAG AA and AAA, at normal size, at large size, and for interface elements. When it fails, it works out the nearest color that passes by moving lightness only, so what you take back to the brand owner is still recognizably their color.
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Large text looks like this
Body copy looks like this. Heat illness is caused by the body’s inability to maintain a normal temperature, and it has three levels of severity.
Small print, captions and disclaimers look like this.
| AA, normal textBody copy, labels, anything under 24px | 4.5 | Passes |
| AA, large text24px and up, or 18.7px bold | 3.0 | Passes |
| AA, interface and graphicsButton borders, icons, chart lines, focus rings | 3.0 | Passes |
| AAA, normal textThe stricter target, required by some public sector contracts | 7.0 | Fails |
| AAA, large textHeadings at the stricter target | 4.5 | Passes |
It measures the contrast between a text color and a background and reports it against the WCAG thresholds: 4.5 for normal text at AA, 3.0 for large text and for interface elements, 7.0 for AAA. The preview shows real copy at three sizes, because a ratio is a number nobody can picture.
When a color fails, it works out the nearest one that passes, and it does so by moving lightness only. Hue and saturation are left alone deliberately. A suggestion that shifts the hue is no longer the brand color and gets rejected the moment you show it to whoever owns the brand. Moving lightness keeps it recognizably the same color, which is what makes the answer usable rather than merely correct.
A pair measuring 4.49 is displayed as 4.4 and marked as failing. Rounding it up to 4.5 would claim a pass it did not earn, and an auditor running their own tool will not round in your favour.
Contrast is one requirement among many and passing it does not make a course accessible. It also assumes both colors are opaque: a semi-transparent text color depends on everything behind it, so flatten it first. And a color that passes on a calibrated monitor still has to survive a projector in a bright room, which no formula accounts for.