Blue images are named fully in the app. At least in gb it was...pretty sure it hasn't changed. Notice typically when u switch... its gray. It turns blue later I believe when its "fully" connected.
Sent from my Galaxy Nexus using DroidForums
As far as I know, the way it works is this:
gray 3g/4g indicator with gray signal bars - phone is attempting to connect to the 3g/4g network, in process of attempting a DHCP lease
gray 3g/4g indicator with blue signal bars - phone is connected to VZW network and has a valid IP. However, device is not fully signed into Google yet
blue 3g/4g indicator with blue signal bars - phone is connected to both VZW network and Google's servers, obviously has a valid IP address