Why thank you Smile
Just to clarify one thing: I don't propose actually having a treaty with Blue. I generally find permanent treaties too limiting to future expansion, so perhaps a time-limit or conditional treaty (a conditional treaty would be something like "I will not attack you as long as Yellow still has Europe") would be in order.
Propaganda-wise, for Yellow and Blue, I would say something like "I'm not really a threat to you, and I won't be unless you threaten me. All I want is my one Risk card per turn." Perhaps I could even work something out with Yellow where we continually trade possession of Ural, leaving one man at the end of our turn, simply for the Risk card. (This would also provide a valuable opportunity for a strike into Ukraine should Yellow become too strong.) Blue depends on my good will to retain South America and probably to stay in the game in any case, so he probably won't be attacking me anytime soon. The European border is much more poorly defended, after all.
I would also point out that attacking me would be a suicidal move in any case, since there are three equally balanced powers at the moment - Red, Blue, and Yellow. The first one to attack will cripple the second, but will in turn be crippled by the third. I will continually remind Yellow to watch his southern border and Blue to watch his African one. I would also refrain from pointing out that Green is bound to upset the balance of power, most probably in my favor - whether he attacks Yellow or Blue, I can hit the other one in the next turn and get both my major enemies out of the way.
For Green, I would simply fortify Kamchatka enough that it wouldn't be a viable option for him to attack. His Middle Eastern army seems poised to strike in any case, so I doubt he would double back and hit me from Asia. He's not a threat to me. Australia has too few armies and is too far away. I'll probably kill him off last.
Until Green disrupts the equilibrium, I'm perfectly content to build up my border forces ("purely for defensive purposes, of course"). I might put them in Central America and point out to Blue that if I had wanted SA, I could have taken it already, explaining to him that he is needed to keep the balance of power (but not that I will only need him until Green hits someone). Also, I would hope that someone invades a continent but doesn't get all the way through (say if Yellow hit Africa, but failed to kill off SA) in which case I would swoop in and take the crippled player's cards.
So that's the idea. Keep everyone at an impasse, quietly build my forces and collect my cards, and once there's an opening, smash into it.