The professional fakers have almost perfected the art of bleaching postmarks on classic China and Hong Kong stamps. The usual postmark on Queen Victorian stamps before 1892 is the B62 obliterator common to all British Colonies in style. It can be found in various shades of blue from pale to blackish blue. Apparently, the paler shades can be bleached with possibly hydrogen peroxides or similar chemicals. The faker would only apply the chemical agent to the postmark itself thus the eliminating the overall bleaching effect to the stamp. I have seen Large Dragons, Shanghai Dragons, Small Dragons, Hong Kong QV stamps altered this way. The most dangerous is the recent find of a Hong Kong 96c Olive Bistre used (found usually with pale blue B62) with the B62 bleached out and re-gummed offering as an umounted mint example! The used 96c bistre costs around US$150 - 300 and the fine mounted mint version could fetch between US$70,0000 to US$100,000! There has never been an unmounted version recorded, thus if genuine, I have no doubt it would fetch much more than US$100k. Hence the temptation is there particularly when there are plenty of used 96c around!