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
Check this palette for readable text.
Paste several HEX colours and see how each one contrasts against the same background.
Palette audit
Contrast against #FFFFFF
#F45B1F3.30:1 · Not AA normal text
#F324783.94:1 · Not AA normal text
#11111118.88:1 · AA normal text
#FFFFFF1.00:1 · Not AA normal text
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.