This is the Heroic cube, where a main keyword is Heroic: Targetting your own creatures with buffs, combat tricks are a main focus. Bombs are avoided, I don't want victory to be defined by who got the biggest bomb, the best card. Instead synergistic decks are encouraged. The idea for the colors is this:
Red: spellslingers
Blue: ETB effects and return to hand
Black: Discard, die effects and returnal from graveyard
Green: Green things, ramp and big creatures
White: last long time to be able to cast big things