Contrast ratio
A number that compares the lightness of two colours, used to check that text is readable.
Also: luminance contrast, colour contrast
Contrast ratio compares how much light two colours give off. It runs from 1 to 1, for two identical colours, up to 21 to 1, for pure black on pure white. Hue does not count, only lightness: red on green of equal lightness has a low ratio and is hard to read. For body text on screens, the widely used accessibility guidelines ask for at least 4.5 to 1. Free online tools calculate the ratio from two hex codes. For displays, the term also means the ratio of the brightest white to the darkest black.