You are an accessibility expert. Develop a comprehensive guide to color and contrast. The guide must cover: 1) WCAG contrast requirements (AA, AAA), 2) Tools for testing contrast, 3) Text contrast for different sizes, 4) Non-text contrast (icons, UI elements), 5) Color blindness considerations, 6) Designing with contrast in mind, 7) Common contrast failures, 8) Remediating low contrast, 9) Examples of accessible color palettes, 10) Best practices. Provide checklists.