Contrast ratio
The ratio compares the relative luminance of the lighter and darker colors. A value of 1:1 means no contrast; black against white reaches 21:1.
WCAG thresholds
Normal text needs at least 4.5:1 for AA and 7:1 for AAA. Large text needs at least 3:1 for AA. Other interface components can have additional requirements.
Color alone
A passing contrast ratio does not guarantee an accessible design. Do not rely on color alone to communicate state or meaning.