[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...