I'm guessing that a production version would have been the same 'high speed gray' that very early P-80As were in--until they noticed that there was no particular adhesion to the aircraft skin at speed... (oh--is an 'extended smiley" twice as long?)
These photos are actually of the manned glider tech demonstrator for the Northrop JB-1 flying wing guided missile, not the XP-79, because this machine did not have a prone cockpit design as on the MX-324/MX-334/XP-79.