Function contrast_ratio 

pub fn contrast_ratio(a: AlphaColor<Srgb>, b: AlphaColor<Srgb>) -> f64

WCAG 2.1 contrast ratio between two colours, in [1.0, 21.0] (1.0 = same luminance, 21.0 = black on white). WCAG AA body text wants ≥ 4.5:1; AAA wants ≥ 7:1.