A blue dot for a user's location indicates that the location data that Zello has for the user is current. On the other hand, a purple dot indicates that the user's location data is outdated. 

A blue dot turns purple if Zello has not received location data from the user after 1.5 times the location tracking interval. For example, if your network's location tracking interval is 10 seconds, then a dot will turn purple if that user's location was reported more than 15 seconds ago. 

You can change your reporting interval by going to Settings > General > Location tracking within the management console. 

Additionally, you can hide locations that are more than 20 minutes out of date using the Hide old locations toggle in the Map settings.