Oh geez. Can't use an FO that evokes the smell of Zombies unless you want it to be a novelty soap that won't get used.
If you want a scent that matches the color, I agree with lime or you could use something like Aloe or ALoe and cucumber. That sounds a little weanie for a zombie soap though- aloe.
I would consider going with something along the neon line. Bright smack in the face green should have a bit of a smack in the face fragrance. That's a bit more subjective for you but for me, a sharp ginger smell comes to mind. Ginger Lime or one of those Ginger combo's can be pretty zingy. So can Gingerale, as long as it's not sweet. Or maybe the FO that smells like those burny cinnamon Red Hots? Or Orange Chili Pepper, but heavier on the chili?
Or you can go earthy, evoking rising from the grave. Won't be as appealing for use for some, but it depends on what you want. Something patchouli based? A mix of grass FO and dirt FO to smell like the earth?
It will be interesting to hear other ideas too. Zombie soap FO. That's a tough one.