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Depends on what it’s controlling. The tune of my car? Red = performance, green = eco-mode.

The valve on a pipe? Red means “stop” (off) and green means “go” (on).

What about a heater? Does red mean hot? Or stopped?

Sibling comments cover colorblind users, but those concerns also apply to most e-ink users as well.

In fact red and green are generally not used for flow in a pipe because they have specific meaning for electrical equipment - green denotes an open circuit and red an energized one. This can be extremely confusing with something like an MOV so convention us to avoid red and green on valve position, pipe fill, etc.

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