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.