Colour contrast

Colour contrast refers to the visual difference between colours, specifically how they are distinguished from one another based on their hue, value (lightness/darkness), and saturation. 

Colour It determines how a colour stands out against its background and impacts both visual aesthetics readability

WCAG 2.0 level AA requires a contrast ratio of at least 4.5:1 for normal text and 3:1 for large text. 
WCAG 2.1 requires a contrast ratio of at least 3:1 for graphics and user interface components (such as form input borders). 
WCAG Level AAA requires a contrast ratio of at least 7:1 for normal text and 4.5:1 for large text.