When that mana is spent to cast a creature spell that shares a creature type with your commander, scry 1
What makes for an interesting deckbuilding exercise, though, is that any and all cards in your deck have to be completely colorless
Each set of showcase cards has been tied to a new Magic mechanic
You'd play cards like Festering Goblin and Noxious Ghoul to wear out opposing forces, utilize Shepherd of Rot and Gempalm Polluter to knock down life, and use powerhouses like Undead Warchief, Soulless One, and Rotlung Reanimator to round out the package
I recommend putting at least one of those two options into your Selesnya deck, for when you absolutely have to put that Jace, Wielder of Mysteries out to pasture
For each flip you win, create a 1/1 red Goblin creature token thats tapped and attacking