Check the WCAG contrast ratio between two colors and AA/AAA pass online, in your browser.
Check the contrast ratio between a text colour and a background and see whether it passes WCAG AA and AAA for normal and large text. Essential for accessible, readable interfaces.
AA needs 4.5:1 for normal text and 3:1 for large; AAA needs 7:1 and 4.5:1. The tool reports each.
About 18pt regular or 14pt bold and above — it gets the easier threshold.