What does protection against green mean in magic the gathering?

Answer 1

Protection From: in Magic the Gathering means a selection of things.

The Protected permanent cannot be damaged by a source it is protected from, nor can it be enchanted/equipped, nor can it be blocked, nor can it be targeted.

Players refer to this as DEBT protection - Damage, Enchant/Equip, Block, Target.

So a creature with Protection from Green cannot take any damage from a Green source, it can't be enchanted or Equipped with a green Aura or Equip (equips are generally colourless but it's not impossible for them to have one), it cannot be blocked by Green creatures, and it can't be targeted by a Green spell or ability.

If however a Green effect does not do any of the above, then Protection will not stop it. If a green spell said 'tap all creatures controlled by target opponent', none of this is covered by 'DEBT' so the protected creature will still be tapped. Note also that non-targeted damage (ie, Hurricane) is still protected against because it deals damage.

Lastly, if a creature/spell is multicolour, everything it does, like damage or targeting, counts as being each of those colours. So if the creature is Green/Red, it is still a Green creature so can't block a protection from: Green creature, nor deal damage to it if itself was blocked by one.