These visual signals are often called Andons, and can take any form that is useful. A popular type is a stack of four lights: Red (for machine down), Yellow (for close to down, such as running low on stock), Green (for running) and Blue (for setup). Andons can also be elaborate digital display boards.
They don’t always have to be visual however. In some of the less-noisy Toyota plants, each production area has an assigned song. If you are the supervisor, if they start to play your song you know you have an issue to attend to!