This is something I noticed on my Android device. When you have a notification, there's a slowly pulsing notification light when your phone is sleeping.
This light comes in different colors, I've seen it as white, blue, and green. This seems to be something that can be controlled by the app that sends the notification. E.g. Facebook is blue, Twitter is green.
Are there conventions as to what the color ought to represent? A quick google search doesn't bring up anything specific.