[sunkencity]

absence of signal is not a good indicator of operation. it is the same signal as total failure of your system.

[dingaling]

Interestingly that reflects Airbus vs Boeing cockpit philosophies.

In the early 1980s Airbus adopted Dark Cockpit where the default state of all annunciator lights is off. If one illuminates then it indicates something worthy of attention. The colour of illumination indicates functional state; blue is good, amber is malfunctional.

[andai]

When would something illuminated blue? When it's both working smoothly and worthy of attention?

[jon-wood]

I assume that's for indicators of abnormal, but safe, state. One example could be the undercarriage being down, a situation that would usually not be expected, but when you're in the process of landing doesn't warrant attracting your attention as an error state.

[michaelt]

According to [1] for the wing anti-ice system, Blue = Turned on, Unlit = Turned off, Amber = Malfunction

Of course, one might very well argue that 'turned on' is not 'worthy of attention'

[1] http://www.smartcockpit.com/docs/A330-A340_Overhead_Pushbutt...