User interface

A User interface allows a user to interact with a machine. User interfaces mainly provide two things:
Many machines can be very dangerous. A machine should have a user interface that can be handled easily, even if the person operating the machine has panicked. The user interface should therefore be intuitive, and simple to use. An example of such a user interface is that of the kill switch. A kill switch must shut off the machine at all costs - the idea is to avoid injury or harm to people. This is very different from shutting off the machine at the end of the shift, or when it is no longer needed.
According to EN ISO 13850, the kill switch has to be red on a yellow background. 
The colors used to mark different states are close to those used by signals used on the road.
There may be additional symbols, for example:
In many cases, such symbols are better, because some people are color blind. They need to be explained, like warnings, though.