Whichever word I repeated in my head as I heard it was what I heard even if looking at another. I could easily and reliably make it change between brainneedle, brainstorm, greenneedle and greenstorm using this method.

Edit: kind of think this is to do with the n at the start of needle being implicit as you already get the n sound at the end of green and brain. I think if you register another n the remaining sound sounds most like eedle and if you don't the remaining sound sounds most like storm... Or I may have listened to this too many times