Protection from blue means the creature cannot be the target of blue spells or blue abilities. This is encoded in CR 702.16b, which states that a permanent with protection from a quality cannot be targeted by anything with that quality. The color of the spell is what matters, not who controls it.
The phrase 'you control' on the spell simply restricts which creatures are eligible targets — it does not override protection. When you choose targets (CR 601.2c), a creature with protection from blue is still an illegal target for a blue spell, regardless of the ownership or control restriction attached to the targeting instruction.
If you attempt to target such a creature anyway, the target is illegal. If the protection exists at the time of casting, you cannot even legally select it as a target (CR 509.1c for during combat, CR 601.2c for spellcasting). If protection is somehow gained after targeting but before resolution, the spell will be countered on resolution by rule (CR 608.2b).
Example: You control a White Knight (protection from black) enchanted with Curiosity — wait, let's use blue: you control a creature with protection from blue and cast Inspired Charge... actually, for a blue example: you cast Favorable Winds-style — concretely: you cast a blue Aura spell saying 'enchant creature you control' targeting your own creature that has protection from blue. You cannot legally choose that creature as the target; you must pick a different creature you control or not cast the spell.
In summary, protection overrides all targeting, even self-targeting or 'you control' restrictions. There are no exceptions based on controller identity (CR 702.16b).
Unofficial fan resource — not affiliated with or endorsed by Wizards of the Coast. Answers are AI-generated estimates grounded in the Comprehensive Rules and are not a substitute for an official judge. Verify anything match-critical.