アクセシビリティメモ
RGB値を用いたコントラストの計算
Checkpoint 2.2 - Ensure that foreground and background color combinations provide sufficient contrast when viewed by someone having color deficits or when viewed on a black and white screen
in "Techniques For Accessibility Evaluation And Repair Tools W3C Working Draft, 26 April 2000"
Guideline 1.4 Make it easy to distinguish foreground information from its background
in "WCAG 2.0 Guidelines"
http://panasonic.co.jp/ud/forum/voice/01/index.html