Does protection from a color prevent the creature from being enchanted by an aura of that color?

Protection from a color grants immunity to four things, remembered by the acronym DEBT: the creature cannot be Damaged, Enchanted/Equipped, Blocked, or Targeted by anything of the stated color (CR 702.16b). Auras that are the protected color fall squarely under the 'E' — enchanted/equipped — part of that rule.

This means two things: first, you cannot legally attach a same-color Aura to the creature as a spell or ability (CR 702.16b, 303.4c); and second, if a creature somehow gains protection from a color after an Aura of that color is already attached, the Aura immediately falls off and goes to its owner's graveyard as a state-based action (CR 704.5n).

Note that the protection must match the Aura's color, not its effect or subtype. A blue Aura that grants a white bonus is still blue, so a creature with protection from blue cannot be enchanted by it.

Concrete example: You control a White Knight (protection from black). Your opponent tries to cast Unholy Indenture (a black Aura) targeting it. The spell is illegal because White Knight cannot be enchanted by black permanents — the spell cannot be cast targeting White Knight, and if somehow it were already attached, it would immediately be put into the graveyard.

If White Knight instead gains protection from black after an opponent's black Aura is already enchanting it, the next time state-based actions are checked (CR 704.5n), the Aura is unattached and placed in its owner's graveyard.