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Redundant Cues

To make color-encoded information accessible to people with color vision deficiencies (a.k.a. color blindness), good design should use redundant cues.  That is, the color-encoded information should also be encoded using some other visual cue, e.g. shape, texture, lightness, etc.  For example, the old Apple Chat interface used a combination of hue (green, orange and red) and shape (circle, triangle, square) to indicate whether potential chat partners were available online:

This duty free store at an airport (I can no longer remember where I took that photograph) uses only red/green hue difference to communicate which checkout lines are open, making this information inaccessible to most people with color vision deficiencies:

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