Since the areas of B.T.H. yelowing tend to be extensive, I would not recommend the use of Spray'N'go, or any other Sodium Metabisulphite based product, due to the fact that the fumes given off are a bronchial irritant and could cause some distress to anyone exposed to the over a prolonged period.
As our Norwegian friend has said, citric acid, through its' mild bleaching effect can be beneficial.
If you do not have access to this, Prochem Oxybrite + Fibrebuff + B105, mixed & used as per celulostic browning instructions, can be very effective.
John.