Posted from the rules section:
Rule #2: No secret diplomacy
Any form of diplomatic discussion between opponents must be posted in the game chat in English or in a language that all opponents understand. Diplomacy includes, but is not limited to: proposing truces, negotiating alliances, and coordinating assaults. Secret diplomacy can be hard to prove, but if you suspect it you should leave the players in question appropriate ratings. If you feel certain about players engaging in secret diplomacy consider reporting them in the Cheating & Abuse Reports forum by posting a new topic.
My suggestion to you would be to ask the other player (yellow) in game chat for an alliance also. That way it's two against two.
![Wink ;)](./images/smilies/icon_e_wink.gif)