The flag is designed to be used by one team, so making it work for either one takes a few entities.
On the beginning of map, the flag switches from Any to Blue to Red back to Any, this activates the directional arrows used by the CTF gamemode. When no player has the flag, it is set to any. Grey arrows point out the flag to all players. When a player picks up this flag, the brushes determine what team he is on, and set the flag opposite to this data. The opposing team sees their arrow pointing to the flag carrier. On dropping, capturing or returning, the flag resets to Any.
I have included an extra brush called triggernotify, this is if you want the flag carrier to receive a message upon picking up the flag. Add a gametext entity and set it up so trigging trigger_notify displays the message.
Most often you are bringing the intel to the enemy base. In the example provided you will see the map is setup like in a regular CTF, all you would have to do is swap the capture zones for this to be a push map.