Protection from a quality grants four specific shields, remembered by the acronym DEBT: the protected creature cannot be Damaged, Enchanted/Equipped, Blocked, or Targeted by sources of that quality. The 'blocked' component means white creatures cannot be chosen as blockers for a creature with protection from white.
This restriction is enforced during the declare blockers step (CR 509.1b). When a defending player declares blockers, they may only assign creatures to block an attacking creature if each chosen blocker is a legal blocker. A white creature is not a legal blocker against a creature with protection from white, so it cannot be assigned to block it at all.
Note the direction carefully: it is the attacking creature's protection that prevents the white creature from blocking it. Protection does not stop the protected creature itself from blocking; it stops others from blocking the protected creature.
Example: Your opponent attacks with a Paladin en-Vec (protection from black and red). You control a White Knight (a white creature). White Knight can legally block Paladin en-Vec because Paladin's protection is from black and red, not white. However, if instead the attacker had protection from white, your White Knight would be unable to block it.
Relevant rules: CR 702.16b defines the blocking restriction, and CR 509.1b outlines the legality checks during the declare blockers step.
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.