Protection has four functions, remembered by the acronym DEBT: the protected permanent can't be Damaged, Enchanted/Equipped, Blocked, or Targeted by anything it has protection from (CR 702.16b). The key word here is damage — protection prevents damage from sources with the relevant quality.
When a fight spell (like Pit Fight or Savage Punch) resolves, each creature deals damage equal to its power to the other creature (CR 701.12a). Crucially, this damage is dealt by the other creature, not by the spell itself. So if Creature A has protection from green, and Creature B (a green creature) fights Creature A, the damage source is Creature B — a green source — which protection would prevent.
However, if Creature A has protection from instants (or protection from the spell's color/type), that protection does NOT stop the fight damage because the damage source is still the other creature, not the spell. Protection from a quality only blocks damage from sources that have that quality. The fight spell merely instructs the creatures to deal damage; it is not the damage source (CR 609.7).
Concrete example: You cast Pit Fight targeting your 3/3 and your opponent's 2/2 that has protection from instants. When the spell resolves, your 3/3 deals 3 damage to the 2/2, and the 2/2 deals 2 damage to your 3/3. The 2/2's protection from instants does NOT prevent the 3 damage, because the damage source is your 3/3 (not the instant spell). The 2/2 takes 3 damage and dies.
In summary: protection can prevent fight damage only if the other creature (the damage source) has a quality the protected creature has protection from — not merely because the fight spell itself has that quality (CR 702.16b, 701.12a).
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.