Another question that just has to do with timings: if I have Diregraf Colossus on the board, and I cast Fleshbag Marauder, can I sacrifice the Zombie token generated by the Colossus' ability whenever a zombie spell is cast? I think I can as I have cast the spell although it hasn't entered yet, but just wanted to check.
As you correctly stated Diregrat Colossus triggers when you cast Fleshbag Marauder, as it is a zombie spell. So the Colossus' ability goes on the stack above the Marauder, so will resolve before Marauder. So you will have your zombie token long before the Marauder resolves and its ability triggers.
No, neither is a zombie spell, they are only spells creating zombie tokens. What you need a spell with the subtype "zombie", usually a zombie creature spell, but can be a tribal spell as well. A Nameless Inversion for example, IS a zombie spell, and would trigger the Colossus.
As you correctly stated Diregrat Colossus triggers when you cast Fleshbag Marauder, as it is a zombie spell. So the Colossus' ability goes on the stack above the Marauder, so will resolve before Marauder. So you will have your zombie token long before the Marauder resolves and its ability triggers.
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