I am probably an innocent, but why do you intend to use an optical brightener?
What we see is the result of light reflection/absorbtion. change in brightness is altering the material
Optical brighteners in spot cleaners tend to lighten the material and in many cases when poorly applied leave those horrible cleaning rings that we have all seen.
Are all brighteners at the bleaching /oxygenator end such as staybrite or do the acid rinses, which all manufacturers appear to claim, also brighten and stabilise colours?
regards
Eddie