@protoaddct Black has almost every permutation of "destroy target creature with restriction X". Destroy target nongreen creature, destroy target white or green creature, destroy target creature with CMC less than 3, destroy target monocolored creature, destroy an untapped creature, destroy a tapped creature, destroy target creature that isn't enchanted, destroy target enchanted creature, destroy target enchantment creature, destroy target spirit, destroy target non-spirit, destroy target human, destroy a non-merfolk creature, destroy a non-elf, destroy a non-elf with power =/= toughness, destroy a cleric, destroy a ninja, destroy target nonlegendary creature, destroy target legendary creature...I think I've made my point.
Destroy/exile target colorless creature was more a matter of time (and eventual need), rather than a restriction imposed by the color pie. As for gaining life, I know we focus on Eternal Formats, but R&D does not. People might think this card is designed for Modern or Vintage or Eldrazi or Shops or whatever, but that's unlikely to be true. Standard has been dominated by aggressive decks centered around Vehicles, Bomat Courier, and Scrapheap Scrounger since Kaledesh. Heck, I don't know what the timing was, but it's possible Smuggler's Copter was still unbanned then. The life gain supports this card's function as an answer to these aggro decks. Adherence to some arbitrary notion of a color wheel is likely secondary to printing functional answers to existing problems in the Standard format.