Zoo is a fun multi-color strategy where you build your deck with the cheapest but most efficient allies available. Normally you only want to use 3 colors, as too many colors weaken the deck's consistency. The best color combo is probably Black, Red, and Green. You can take one of the three colors out of the deck and successfully add Yellow instead. You want to play as many allies as early as you can and clear the way for them with cards like Fire Arrow, Swamp Witch, and Vampiric Bite. Your goal is to have your opponent dead before a Killer Wave is played. If you fail that's not a problem, since your Cave Trolls will live and you just have to continue playing threats. Don't forget that this deck is about the quality and cost of your allies, what little else you put in your deck should only help you get the job done.
A list of good efficient-cost allies includes: Evanescent Pyro, Horrid Skeleton, Cave Troll, Mad Barbarian, Skeletal Warrior, Soulseeker, Swamp Witch, The Haunter, Winged Imp, and Squire because it costs zero and requires no energy. Everything should cost 3 or less, though Great Bear works well too.
Here's a great combo: if you play Evanescent Pyro followed by Hungry Ghoul and consume your Evanescent Pyro, you will end up with a permanent 5 AP/ 6 HP ally. This can be done as early as your second turn. This combo is not necessary for this deck, but can be brutal if done early enough.
This deck is fun and has lots of options, which means it can be built in many ways and the versatility will be almost endless. It can be tricky to build a three color deck, especially if you are new to orbs. Keep the colors in near-even amounts, something like 16/17/17. Once you get the hang of multiple colors feel free to switch it up.
Let's break the colors down for you:
Red: Red has fast and cheap allies and actions that can damage allies or players, not to mention Orb of Hostility for some added fun. Remember to keep this deck simple as it requires a large creature base and there is not a lot of room for other cards.
Green: Green has access to Fresh Force. Green also has allies that are hard to deal with, like Cave Troll and Great Bear; it also has Ant Bandit which is easily dealt with normally, but if you have put a lot of pressure on your opponent early game, in the late game the Ant Bandit may live because your opponent's hand is exhausted or your opponent's life total is so low they have to block a more dangerous threat.
Black: Some of the most efficient allies are found in this color. Most do require you to remove an ally in your graveyard from the game, but this is no problem for a deck built like this one. You also have Vampiric Bite and Skull Ring to play with. Please remember you do not have much room for cards outside of the ally type.
Yellow: If you choose to swap a color for Yellow than you will see the deck play a little bit different. Yellow adds Energetic Wizard, Sneaky Knight, and Telekinesis. With the addition of Sneaky Knight you may want to add the addition of Grow Big especially if you still have Winged Imp.
Weak against: control decks, especially ones that run board sweeps like Killer Wave.