Iridian Maelstrom

The joke is spelled out in the mana cost. To cast a sweeper that spares only the WUBRG-colored, you need every color represented at once, which is precisely the ecosystem the effect protects. The board wipe becomes an asymmetric one the moment you can produce all five colors and stock the battlefield with a five-color creature or two; every mono-, two-, three-, and four-colored body on the table dies, and only the truly rainbow survive. That "isn't all colors" clause is doing sharp work: it does not read "multicolored" (which would let a Boros bird live) but demands the full pentacolor stamp, a rarity so specific that in practice the card functions as a near-total board reset with a designer-placed loophole for the one deck built to walk through it. It sits in the lineage of the one-sided Wrath, the sweeper that reads clean and symmetrical until you notice whose creatures it names. Where a color-hate wrath punishes a hue and a converge-style effect scales with your investment, this one rewards a very particular deckbuilding thesis: assemble the five-color engine, and the wipe you'd normally hold back becomes a button you can press with impunity. The elegance is that the exemption is not a keyword or a tribe but the same requirement the spell already levied to be cast, folding the restriction and the reward into a single line.