You might try Ektamatic (AKA Chromoskadisic) stabilizer - it is used to preserve the color of 'lumen prints'.

100gm Am. Thiocyanate
35gm S. Metabisulfite
30ml 28% Acetic Acid
water to make 500ml
dilute 1:4 for use

http://www.apug.org/forums/groups/lu...ped-color.html
http://www.apug.org/forums/groups/lu...giv-wt-rc.html

Dilution and timing aren't critical. Purple is the hardest color to preserve.