Contrast ratio
Contrast ratio compares the relative luminance of two colors on a scale from 1:1 to 21:1, and it determines whether text on a background is legible.
WCAG AA expects at least 4.5:1 for normal text and 3:1 for large text, and only measurement settles it—equal lightness values do not imply equal contrast. Continue with Color and contrast: making palettes readable for the longer explanation.