Can you target a creature with protection from white with a white aura if the aura itself is not white?

Protection from white prevents a creature from being targeted by anything that is white (among other things). When you cast an Aura spell, the targeting happens as part of casting the spell, so the color of the Aura spell itself is what matters — not the enchantment subtype or any other characteristic.

The four things protection prevents are neatly summarized by the acronym DEBT: the protected permanent cannot be Damaged, Enchanted/Equipped, Blocked, or Targeted by anything with the relevant quality (in this case, white). CR 702.16b lists these restrictions explicitly.

So if you have a white Aura — meaning the Aura card itself has white in its color identity as printed or granted — you cannot legally target a creature with protection from white when casting it. The spell would be an illegal target and cannot be cast that way. (CR 601.2c requires all targets to be legal at the time of casting.)

However, if the Aura is a different color — say a green Aura that happens to grant a white effect — then protection from white does NOT prevent it from targeting the creature, because the Aura spell itself is not white. The condition that matters is the color of the Aura, not what it does.

Example: You try to cast Pacifism (a white Aura) on an enemy creature with protection from white. This is illegal — you cannot choose that creature as a target because Pacifism is a white spell. Contrast this with casting Capture Sphere (a blue Aura) on the same creature: that is perfectly legal because the Aura is blue, not white.