Smithy's Factory Fog?

[JackMan]

I guess it's fog.. Smoke, maybe? I don't really know what it is, but I want it.

http://www.bghq.com/bgs/snes/s/smrpg/702.png

You can see it sort of clearly in the blacker spots. It's purple. I have no idea how people get the upper.. layer of the sprites, but you guys have done it before, and if you could do it again it would be much appreciated.
Maybe you could use some sort of emulator to block out the other layers, I don't know.
I'm just saying it would be much appreciated if someone could get the fog for me.

Thanks anyway.