Web Accessibility Policy of the State of Maine
3.1 - Do not convey information with color alone.
Color is often used to indicate special functions or status. For example, required form fields are frequently indicated with red labels.
Users with blindness, limited vision, or color-blindness may miss information presented with color.
Whenever color is used as an indicator, use a non-color-based indicator as well. For example, required form fields could be identified with asterisks as well as color.
Ref: WCAG 2.1; 508 c
3.2 - Use contrasting foreground and background colors.
Web authors can set specific colors to be used for foregrounds (text) and backgrounds. Sometimes images are used as backgrounds.
Users with limited vision or color-blindness may have difficulty reading text that is similar in color to its background.
For text, use dark colors on light backgrounds, or vice versa. Avoid combinations of red and green as well as busy background images.
Ref: WCAG 2.2