As far as I know, a brown airport on the low en-route chart does not have an approach procedure but blue and green airports do. But what exactly are the differences between these colors?
From the FAA Aeronautical Chart User's Guide:
Facilities in BLUE or GREEN have an approved Instrument Approach Procedure and/or RADAR MINIMA published in either the FAA Terminal Procedures Publication or the DoD FLIPs. Those in BLUE have an Instrument Approach Procedure and/or RADAR MINIMA published at least in the High Altitude DoD FLIPs. Facilities in BROWN do not have a published Instrument Procedure or RADAR MINIMA.
So the green ones are in the FAA TPP but not in the DoD High Altitude FLIPs.