I'll cut to the chase first and add detail if needed
I applied some ascorbic acid (in the form of crushed Vit C tablets) to a couple of discolored areas in my pool (the discoloration is over most of it but this was just a test). The areas that were treated all took on a grey discoloration that disappeared (along with the original stain) when I added chlorine (superchlorination).
The conclusion I have drawn is that yes, the discoloration is metal based but what I am not sure about is whether the grey discoloration was just a result of me not having a sequestering agent in there to suspend the released metals and if it was, why did chlorine appear to remove it?
Any insight is greatly appreciated.