Digging into previous formats, there were decks that centered around Salamence Lv. X and a Flygon from a DPPt era set that were absolutely brilliant cards, and those two were actually of the Colorless type. Though those worked best with other types of Pokemon in them, since Salamence LV X's energy requirements were heavy on the Water and Fire type energy, and that particular Flygon had a PokeBody called "Rainbow Float" that made a Pokemon that shared a type with a Basic Energy attached to Flygon have a retreat cost of 0.
It's certainly possible to go full colorless, it's just usually not an optimum route, depending on what's in the deck and what it has to compete with.