Can I use the green-red wire from the voltage regulator as 12 volt source?

I've used the red/grn wire off the voltage regulater to run the hot wire on a Pertronix ign conversion for the distributer, or the electric choke on a Holly carb. Great switched 12v source, but that wire is not heavy enough to run any high current devices like a fan. You could use it to run a relay that runs the fan, but I don't think that was what you were after. A fan would overload this circuit very quickly I would think.

Oh, the brown wire runs off the ballest resister wire from the ign switch. So it's current limited. That why its a bad choice for the electric choke and the Pertronix. I have used it but not recomended and i don't like splicing at the solinoid. Always looks goofy no matter how you splice it but thats just me.

Yes, if you're using it to trigger that relay it should work fine.