Those rings
You can take multiple mulligans, but when decide to keep your hand, you have to return one card for each mullidan you took
So in this case, Blue is stronger than Black in which I have heavily invested
So Charizard EX decks are packed with cards that let you draw cards (like Pidgeot ex), search for cards (like Supporter Arven or Ultra Ball), or speed up evolution (Rare Candy)
These are generally blue-green decks and a hyper-specific subcategory of control decks
Whispers of the Muse (T1, Ext, S, Block) A favorite in "Turbo Xerox" (a blue deck which used minimal land but a ton of cantrips to thin out the deck), Counter-Phoenix (Shard Phoenix + Forbid) and Tradewind-Awakening (put Awakening on the stack, tap all your lands for mana, untap all your lands), Whispers has what might seem like a hefty price to pay for one card but you get to do it turn after turn after turn