Easy, you pick white!Well, the problem with choosing only one colour is that one is often lacking in one or more departments (e.g. enchantment destruction for red). Both single colour cube as all colours but super lands have their own design challenges
It has aggro support, midrange support, control support, targeted removal, mass removal, life gain, counterspells, discard, direct damage, ramp. It does everything!
It also happens to supports several archetypes very well; spells matter, artifacts matter, enchantments matter, go wide strategies, equipment matters, various tribes like Kithkin and Soldiers.
Aaaaaand I see I was sniped by @LadyMapi