However, other colors can get in on the action too
Light allows UW to answer troublesome permanents like Esikas Chariot, Meathook Massacre, or The Celestus without needing to sweep the entire board with Mastery or (now) Farewell
The biggest thing to learn are which abilities you can trigger so here are a few great examples of allies with text that includes when, whenever, and at: Katara, Bending Prodigy, South Pole Voyager, Ondu Cleric, Earth Kingdom General, and Hero of Goma Fada
In short, you want to balance the mana value of your spells to ensure you have effective spells to cast throughout the game
Faerie Swarm could be a huge creature for only 4 mana if you can manage to get a lot of small blue faeries or other blue permanents onto the board