Ally Decks are great because most of them work together to make each other stronger which is why I love to use it! Now, my deck has all colors in it but you can definitely make a deck with only 2 or 3. Although I obviously recommend allies that gain +1/+1 counters, they are very easy to find so I won't go through those. I will on the other hand, give you some ideas for other allies that will help in a different way.
When Joraga Bard ( green ally ) enters the battlefield, ally creatures you control gain vigilance until end of turn. Seascape Aerialist ( blue ally ) allows your other ally creatures to gain flying, Highland Berserker ( red ally ) allows your other allies to gain first strike, and Kabira Evangel ( white ally ) allows them to gain protection from chosen color. These are all "until end of turn" affects but in a lot of circumstances, that is all you need for a great attack!
This is where it gets a bit interesting! An army of allies that get +1/+1 counters can only get better by getting them stronger while you wait to cast your next ally. This means proliferating! If you don't know, proliferating allows you to add a counter to any number of spells that have counters on them already. You can choose which ones it affect, either yours, you opponents, or both. Also, there are instant and sorcery cards that give target creature a +1/+1 counter such as the Feral Contest card. Having some of those are good also.
