Friday, July 26, 2013

Colors & Contrast

Notes from the July 25 meeting of the AccessibleWeb@U interest group


  • Presenter - Rick Ells UW-IT
  • Vision
    • Normal - Has red, green, and blue sensors
    • Protanopia – Red/green color blindness, less sensitive to red light
    • Deuteranopia – Red/green color blindness, less sensitive to green light
    • Tritanopia – Blue/ yellow color blindness, confuse blue with green and yellow with light violet
    • Cataracts
    • http://www.color-blindness.com
  • Color Combinations
    • Use orange only for large text or highlight
    • Avoid red on green or green red
    • Avoid using red and green to convey information, such as red text is a warning, green text is a recommendation
    • Avoid putting bright colors at the opposite end of the spectrum together
    • Avoid putting text on textured backgrounds
  • Contrast and Luminosity
    • Contrast is the difference in brighness (luminosity) between two colors, such as between the color of text and the color of the background
      • More contrast is needed for smaller text
      • 4.5:1 for content text (Level AA)
      • 3:1 for large text (Level AA)
  • Guidelines
    • 1.4.3 Contrast (Minimum): The visual presentation of text and images of text has a contrast ratio of at least 4.5:1, except for the following:
      • Large Text: Large-scale text and images of large-scale text have a contrast ratio of at least 3:1;
      • Incidental: Text or images of text that are part of an inactive user interface component, that are pure decoration, that are not visible to anyone, or that are part of a picture that contains significant other visual content, have no contrast requirement.
      • Logotypes: Text that is part of a logo or brand name has no minimum contrast requirement.
  • Example Site
    • http://staff.washington.edu/rells/accessibility/contrast/low.htm
    • Evaluate the example site using the Colour Contrast Analyzer - http://www.visionaustralia.org/business-and-professionals/digital-access/resources/tools-to-download/colour-contrast-analyser-2-2-for-web-pages
  • Maintaining Hue
    • When you need to darken or lighten a hue, maintain the same relative ratios between the red, green and blue values.
    • Example: The following are the same hue, but different luminosities
      • RGB 50, 75, 100 - darkest
      • RGB 75, 112, 150 - medium
      • RGB 100, 150, 200 - lightest
  • Tools
    • Contrast-A http:// www.dasplankton.de/ContrastA/
    • Color Oracle - http://colororacle.org
    • WebAim Contrast Checker - http://webaim.org/ resources/contrastchecker/
    • ColorHexa - http://www.colorhexa.com
    • Colour Contrast Analyzer - http://www.visionaustralia.org/business-and-professionals/digital-access/resources/tools-to-download/colour-contrast-analyser-2-2-for-web-pages

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