on chat:
 if {%player%.Blue} is false:
  cancel event
  broadcast "&1[Blue]&f%player%: %message%"
 if {%player%.Red} is false:
  cancel event
  broadcast "&4[Red]&f%player%: %message%"

command /setteam <text>:
 trigger:
  if arg-1 is "blue":
   set {%player%.Red} to true
   set {%player%.Blue} to false
  if arg-1 is "red":
   set {%player%.Blue} to true
   set {%player%.Red} to false