No mate green (or black for that matter) cannot target Mirran Crusader at all whether it be your or an opponents spell or ability, so he can't be enchanted by green or blocked by green or killed by things like Beast Within and he can block green without any fear or being killed in battle but he can only block up to his toughness of things that have trample so if you block a 9/9 he can block only 2 of that damage before the rest tramples over you. he will not be killed in this process
hope this helps someone usually beats me to these posts lol but im at work so the official rules are not open to me but this should do it for you
Here's the official rules.
702.15a Protection is a static ability, written "Protection from [quality]." This quality is usually a color (as in "protection from black") but can be any characteristic value. If the quality happens to be a card name, it is treated as such only if the protection ability specifies that the quality is a name. If the quality is a card type, subtype, or supertype, the ability applies to sources that are permanents with that card type, subtype, or supertype and to any sources not on the battlefield that are of that card type, subtype, or supertype. This is an exception to rule 109.2.
702.15b A permanent or player with protection can't be targeted by spells with the stated quality and can't be targeted by abilities from a source with the stated quality.
702.15c A permanent or player with protection can't be enchanted by Auras that have the stated quality. Such Auras attached to the permanent or player with protection will be put into their owners' graveyards as a state-based action. (See rule 704, "State-Based Actions.")
702.15d A permanent with protection can't be equipped by Equipment that have the stated quality or fortified by Fortifications that have the stated quality. Such Equipment or Fortifications become unattached from that permanent as a state-based action, but remain on the battlefield. (See rule 704, "State-Based Actions.")
702.15e Any damage that would be dealt by sources that have the stated quality to a permanent or player with protection is prevented.
702.15f Attacking creatures with protection can't be blocked by creatures that have the stated quality.
So basically everything maiden77 said just in rules form.
There's an acronym that Wizards Customer Support taught me when I asked about Protection abilities. The acronym is DEBT, and it stands for Damage, Enchant/Equip, Block, Target. In this case, it means Mirran Crusader can't be Damaged, Enchanted or Equipped, Blocked, or Targeted by anything that's black or green. DEBT has been really helpful to me for remembering how protection works. I hope that wasn't redundant!
Actually, the damage from a green trample creature does not trample over Mirran Crusader. You are confusing it with indestructible creatures.
Trample means that if a creature would dealenough damage to its blockers to destroy them, you may deal the rest of the damage to the defending player/planeswalker. But because damage cannot be dealt to the Mirran Crusader by a green trampler, all damage is prevented.
Actually Trample says "If this creature would ASSIGN enough damage to its blockers to destroy them, you may have it ASSIGN the rest of its damage to the defending player or planeswalker." "ASSIGN", NOT "DEAL".
In the Combat Damage Step, creatures first ASSIGN damage to the creatures they are blocking/blocked by, then all damage is dealt simultaneously. Combat damage doesn't use the stack (anymore).
The ruling on Protection says that damage cannot be DEALT to a creature with Protection from a quality of the source, but it says nothing about ASSIGNING damage.
Mirran Crusader blocks Primeval Titan . The Titan's controller assigns 2 damage to the Crusader (enough to kill an X/2) and 4 damage to the defending player. Then, when the damage goes to be dealt, the 2 to the Crusader is prevented, but the 4 is still assigned to the defending player, and there is no reason it should not be dealt. Defending player takes 4.
(Sorry about using all capitals in some words, but I really wanted to get emphasis across. There's no way to use bold type on this site is there?)