There are 3 armies, not 2 - if you really want to keep track:
Red - Attacking Army for attack fighting - most venues use this (GE 1-4, continent map, PvP, GbG, and of course, GvG - may she rest in peace
Blue - City defenders and GE 5, you control them in GE 5, elsewhere, they defend your city via computer AI responses to city attack
Orange - PvP Arena - specialized for that venue, AI controlled.
In fact, it gets even more granular (specific), under the 3 color system: Most event buildings are now delivering boost value by both color AND venue it's going to work in.
For example: In the next event you'll play here in live, the Celtic Tavern (base) continues the design of venue specific boosts.
It has city defender boost that turns on ONLY when you fight in GE 5.
SO they are getting very specific where things are going to work. Get used to it