With heat applied, the color should go from green to brown. If you apply too much heat, the wood will scorch. It doesn't sound like you've applied so much heat that you've scorched the wood yet, so I wonder if you've stopped short of turning the aqua from green to brown.
I suggest you apply the heat on your scraps until you can see the wood scorch. That way, if the color goes from green to burned, you will know that something is not quite right with the aqua fortis. If this is the case, you should get a different bottle of aqua.
My heat gun will eventually scorch the wood on LOW setting, but the color will change before it gets to that point. I think I mostly use Track of the Wolf brand, if that's what it's called. And come to think of it, I believe it's called Ye old Something. Had good results with it. Good luck. Bill