Well, some things that may draw you into specific colors are: A really strong rare you're excited about playing Having plenty of "removal"cards that can destroy, exile, or keep an opponent's card tapped Opening many cards in that color A good "mana curve" in that colorcreatures of different mana values, more on the 2, 3, or 4 end than on the 5 or greater end Several multicolor cards in those colorscards that have multiple colors are often more powerful than those with just one color Ideally, the colors you pick will have all four of those features, but if only two or three of those are true, that's plenty good

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When a Craterhoof Behemoth wont get the job done, we can instead go and fetch Atraxa, presenting a powerful threat and refilling our hand
Mystic Forge Mystic Forge is a recent printing and represents a powerhouse option to find with Karn