Does protection from a color prevent a creature from being the target of an aura of that color already attached to it?

Protection from a color has four specific effects, remembered by the acronym DEBT: the creature can't be Damaged, Enchanted/Equipped, Blocked, or Targeted by sources of that color (CR 702.16b). The 'T' part stops an aura of that color from targeting the creature when it is being cast or attached through some effect.

However, the question here is about an aura that is already attached to the creature when it gains protection. In that case, there is no targeting event happening — the aura is simply sitting there. Instead, state-based actions handle the situation: CR 704.5n checks whether an aura is attached to something it couldn't legally enchant, and if so, the aura is put into its owner's graveyard.

So the aura doesn't 'target' the creature again once it's already attached. It simply ceases to be legally attached the moment protection is gained, and the game removes it via state-based actions the next time those are checked — not through the targeting restriction of protection.

Concrete example: Your opponent controls a creature enchanted with Rancor (a green aura). You cast a spell that gives that creature protection from green. Before anyone receives priority again, state-based actions are checked: Rancor is now attached to a creature it can't legally enchant, so Rancor goes to its owner's graveyard (and then its triggered ability returns it to their hand). The creature was never 'targeted' by Rancor in this process.

In summary, protection's targeting shield is irrelevant here; it's the 'can't be enchanted' part (the 'E' in DEBT) combined with state-based actions (CR 704.5n) that removes the already-attached aura.