Does then enchantment rooftop storm completely reduce the summoning cost for zombies to 0 or do you still have to pay the colored mana in order to get them out?
Rooftop Storm doesn’t actually reduce or increase what the mana cost is for Zombie spells. Rather, Rooftop Storm gives you an alternate cost you may choose to pay instead of the Zombie’s casting cost. In this case, that alternate cost is (0).
Similar cards that use alternate costs include Abolish, Blazing Shoal and Bringer of the Blue Dawn. You can always choose to pay the regular casting cost for these cards if you prefer.
In contrast, check out cards that actually do reduce the casting cost for spells — such as Ragemonger, Arcane Melee or Blasphemous Act. These effects can’t reduce the colored portion of a spell’s mana cost unless they reduce the cost by that specific color.
