Protection from a quality (in this case, white) grants four protections remembered by the acronym DEBT: the creature cannot be Damaged, Enchanted/Equipped, Blocked, or Targeted by anything with that quality. A white spell that says 'target creature gets +1/+1' is a white source attempting to target the protected creature, which is explicitly forbidden (CR 702.16b).
When you choose targets for a spell or ability, you must choose legal targets. A creature with protection from white is not a legal target for any white spell or ability. If you attempt to target it, you must choose a different target. If no other legal target exists, you cannot cast the spell at all (CR 601.2c).
Even if somehow the spell were already on the stack targeting that creature (for example, if the creature gained protection in response), the spell would be countered by the game rules when it tries to resolve because its target is now illegal (CR 608.2b).
Example: Your opponent controls a Beloved Chaplain (which has protection from white). You try to cast Inspired Charge (a white instant) targeting it to give it +1/+1. You cannot — Beloved Chaplain is not a legal target for your white spell, so the spell cannot target it at all.
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.