Soaking US Stamps Off Paper

I know this has been mentioned before, but for some reason I cannot find the post.

Anyway, I've been going through hundreds of on-paper stamps given to me and have just been going through the process of soaking them off paper. Surprisingly, with most foreign issues, I've had virtually no problems. However, with US issues, I have a number of them that were postally used and cut off of manila envelopes. For some reason, the soaking process takes the manila color off of the envelope backing and yellows the stamps. (I suspect it has something to do with the dye used in some of the cheaper manila envelopes.)

Outside of simply sorting the stamps before soaking, is there a method collectors use to eliminate the yellowing problem or is it just destined to happen? I seem to recall someone suggested a drop of dishwashing liquid might help (?).

Any comments would be appreciated.

I usually don't bother soaking any stamps off coloured paper.
Many years I had some red coloured paper in a pile I was soaking
which discoloured most of the stamps.
I was so mad that I dumped the whole bunch.
Since then I make sure there is no coloured paper included.
Same for stamps fastened with scotch tape. Garbage they go.

I think this is the link you're looking for http://www.virtualstampclub.com/2009usnew.html

Better yet, click on the SEARCH button at the top of this page and use UNSOAKABLE as your keyword

KirkS