Having now read many of the posts, I'd add that the best way to deploy indicators will depend on the application and the situation in which they're installed.

Having now read many of the posts, I'd add that the best way to deploy indicators will depend on the application and the situation in which they're installed. My key point is that if the system isn't fully automatic—that's to say where humans are involved in the process—designers must always consider human ergonomics. Simply put: indicators should be so configured so as to optimise the 'coupling' between the equipment and human observers. This not only goes for lit indicators, bulbs, LEDs etc, but also dials and gauges, etc. For example the color of the dial and the ballistics (damping/settling) in audio VU (level) meters is closely defined. Moreover, although the VU meter has been around for many decades, dispute still surrounds it in that many prefer the PPM (Peak Program Meter) as pioneered by the BBC. Even the color of the dial was changed to the traditional aircraft white on black. I point this out because engineering with human ergonomics at its center is often far from being either obvious or simple. Another example is the analog multimeter versus a digital one. A digital readout meter will give you precision but an analog one with the correct ballistics (usually with critical damping) is much better for nulling or following a drift, trend or rate of change.