Some things that may draw you into specific colors are: A really strong rare you're excited about Plenty of "removal" cards that can deal with your opponent's creatures A lot of playable cards in that color A good "mana curve" in that colormeaning lots of creatures of different costs Ideally, the colors you pick will have all four, but if two or three of those are true that's plenty good
Sideboarding it out needs a good reason
Outlaws, treasure, counters, one of effects, little goad, more
Recently, cards that offer typal bonuses to more than one creature type have become more common
Whenever you attack with three or more creatures, creatures you control get +1/+1 until end of turn
The standout strength here is mana-to-value conversion