Sulfite has quite an effect on dye - but it seems it is the pH change that causes dyes to fade. Sulfite is often added to writing paper to make it “acid free”. The resulting alkaline paper can make fountain pen writing disappear. The effect is reversible: make the paper acid again and the color returns. Of course, then the paper rots away. A demonstration of the effect, using ink in water rather than paper: http://www.fountainpennetwork.com/forum/topic/231307-effects-of-ph-on-blue-black-inks/ I am under the impression that Kodak HCA is buffered to a specific pH - the ‘isoelectric’ point (and here my knowledge runs out). At this pH the gelatin no longer binds to other chemicals (hydrogen bonds?) and this helps with washing out fixer and the purple dye. The purple dye isn’t destroyed, it just washes into the HCA - which turns a brilliant purple. Any remaining purple goes into the wash water.