WCAG
The international guidelines for making websites usable by people with disabilities, including colour rules.
Also: Web Content Accessibility Guidelines
The Web Content Accessibility Guidelines are published by the World Wide Web Consortium and are referred to in the laws of many countries. Several of their rules concern colour. Text should have a contrast ratio of at least 4.5 to 1 against its background, or 3 to 1 for large text, to meet the middle level, called AA. The strictest level, AAA, asks for 7 to 1. Information must not be given by colour alone, so that people who cannot tell certain colours apart are not shut out.