Each colour is associated with a strategy: Steel hits hard, Sapphire is slow and strategic, Ruby hits fast but weak, Emerald is about thinking beyond the playing field, Amythest nerfs your opponent and Amber heals and buffs your deck
Uncommons (U) introduce more specialization and complexity, often signaling key strategies
Arena begins with a relatively meaty tutorial, where you play a few rounds of the TCG as a Navi-like fairy takes you through a beginner's guide to Magic: The Gathering
The rarity of the Enchanted type makes it difficult to pull even one of them from random booster packs, but also means they are quite valuable and special when someone does
I find it quite amusing that it works with other copies of Niv-Mizzet
The conditions are that you cannot have any other creatures on the battlefield or in your graveyard with the same name