Most people's spike hair is their natural hair.
A bunch of people who cosplay spike don't do so very well, or elaborately.
So, what I would do for the wig is get a black wig, and rough it up a bit. Get some spray, and fix it so it looks like his hair.
If you really REALLY feel the necessity to tint it green (I don't think it's green enough to have to do that, but whatever floats your boat) I'd use the crap-o-la spray junk. It'll tint it enough so it wont look REALLY green, but it'll work to make you happy.
That's great Nikki <3
I'll look for it!