Accessibility is when the needs of people with disabilities are specifically considered, and products, services, and facilities are built or modified so that they can be used by people of all abilities. 

Links:

Guidelines_for_Accessibility.pptx (live.com)

ADA Accessibility Standards (access-board.gov)

ADA Standards for Accessible Design | ADA.gov

Online Design/Functionality The Must-Have WCAG Checklist | Level Access

Guidance on Web Accessibility and the ADA | ADA.gov

Law, Regulations & Standards | ADA.gov

Resources: 

Contrast Checkers:

Color Contrast Accessibility Validator (a11y.com)

Color Contrast Checker for WCAG & APCA. Analyse, preview and get color suggestions.