What are the differences between blue airports and green ones on the low en-route chart?
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.