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Because some people cannot distinguish colors reliably, we use redundant cues to ensure that any color-encoded information is also accessible through some other modality.  For example, in most countries, traffic lights are stacked vertically, with red on top and green at the bottom so that even people who cannot tell red from green know how what to do.  In GMail's "Message blocked" automated reply, the red light is shown at the bottom.  People with normal color vision will get the message: "Stop!"  But people with some color vision deficiencies will see it as a "Go"!  This is an example where redundant cues instead of reinforcing the color-encoded information, contradict it instead.

red traffic light next to Google's "Message blocked" message, in which an icon in the shape of traffic light shows a red light at the bottom
In Google's "Message blocked" reply, people who cannot see color will get the opposite meaning from those who can. Photo credit: Kevin Payravi, Wikimedia Commons