Protective Sphere

The activation cost names a color through the mana you spend, and the prevention shield it raises matches that color and nothing else. Pay white, and you switch off a white source for the turn; the colorless clause exists to slam the door on artifact mana or Wastes-style fuel buying the effect for free. That gating is the design problem the card solves: prevention this repeatable would be oppressive as a blanket answer, so it is deliberately myopic on two axes at once. It is blind to whichever color you did not pay for, and, just as important, it covers only one source per activation: the ability picks a single source of your choice, not every same-color attacker on the board. So the cost scales twice over. Facing a mono-red aggressor swinging with five creatures, you pay one mana and one life per source you want to neutralize, firing the ability again for each additional body. Facing a multicolor team, you also have to spend mana of the matching color each time, so the breadth of the threat range and the number of attackers both push the tax up together. It plays like a slow, fiddly, color-keyed fog you can stack on individual sources, and the per-source, per-color restriction is the load-bearing piece of the design: it keeps a would-be universal shield expensive enough that the wider and deeper the threat, the more it bleeds you to hold.