It turns colour into a number.
Colour tools turn visual choices into measurable values such as contrast ratios or colour coordinates. That makes them useful checks, not automatic design decisions.
Colour & Accessibility
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Useful to know
Four things worth knowing.
Passing is not the whole design.
A passing number cannot judge hierarchy, glare, display calibration, print stock or whether the surrounding design is actually readable.
Check it at real size.
Use the number as a signal, then look at the real text size, weight, background and surrounding colours.
Treat the number as a floor.
Check the real size and context after the number passes. Small text and thin weights usually need more breathing room than the minimum.