Elementalist's Palette

Most mana rocks answer the color question. This one answers a scaling question instead, and it does so by inverting the usual relationship between ramp and payoff: the more X spells you cast, the more mana it produces, but that mana is fenced to X costs only. That restriction is the whole engineering feat. A rock that added colorless mana usable anywhere, growing two counters per X spell, would trivialize every fireball and hydra in the deck; by locking the charge-counter mana to costs that contain X, the design lets the ceiling run high without letting the payoff leak into ordinary spells. It becomes a dedicated fuel line rather than a general accelerant. The tap-for-any-color mode keeps it playable early, so the artifact does real work before its counters accumulate, then swells into a battery once an X-heavy board has established itself. The subtle wrinkle is that the counters trigger on casting, not on resolution: a countered Comet Storm still charges the Palette, and the counters persist, so the engine is durable across a game where individual X spells get answered. This rewards a deckbuilding commitment made in advance, paying you for a critical mass of X spells rather than slotting into anything. In a deck without them, the second mode simply lies dormant while the any-color tap earns its keep; in a deck assembled around them, it outgrows every other rock on the table.