Paul,
Firstly, most de-browning agents are also effective at removing coffee stains.
As a second point, I would assume that in your pre-inspection you uplifted a corner of the carpet to check the structure and would at that time determined whether there was any cellulostic material in the structure. If there is none, then cellulostic browing is not the cause of the discolouration.
In this particular instance it is unlikely that there will be colour loss due to the de-browning agent, in view of what has already been done, but a minimum pre-test dwell time of 20 minutes needs to be allowed for these products.
