Broad bean water went brown overnight We had broad beans last night (in a risotto - yum!)

I emptied the beans from their pod, blanched them, then removed the skins from the individual beans before putting them into the risotto.

I never got round to cleaning up the pan in which I had blanched the beans, and when I came into the kitchen this morning I discovered that the water in it was a brown-ish/orange-ish/bronze-ish colour (still entirely transparent, though). There was no colouring at all after I'd blanched the beans, and I didn't do anything to the pan in between - just left it on the hob.

Does anyone know what the colour is, and by what mechanism it appeared overnight?
Speculating - (colloidal) rust - from iron in the beans - via oxidation.