Rending Volley

The color restriction is the whole transaction. By promising never to point at anything outside white or blue, the card buys a damage figure that overkills almost every creature those colors care about, and it does so at the cheapest possible price. Four damage for one mana buries a control finisher, a flying threat, or the centerpiece of an aggressive white deck before any of them can matter. The "can't be countered" clause is the sharper half of the design, and it targets blue by implication. Blue is precisely the color built to hold up permission at instant speed to protect a key creature; this line takes that defense off the table. The spell still goes on the stack like any other instant, but a counterspell aimed at it simply does nothing, so the very deck built to answer removal has no answer to this one. That makes it a clean out against permission-based decks, the decks that expect their answers to be answerable. The lineage is the family of color-locked hosers that trade universality for certainty against their prey, the way protection abilities and narrowly-worded hatebears do: undeniable against two colors, inert against everything else. Outside the white-and-blue gauntlet the card is a dead draw, and that dead draw is exactly what the rate is paying for. Against those two colors, it reads closer to a guarantee than a removal spell.