Red Cabbage Indicator colour change with Sodium Metabisulphite
http://mylespower.co.uk/2012/04/06/homemade-ph-indicator/


Now we add the reducing agent (model compound).


That lessens conjugation and bleaches the anthocyanine.  "But the colour it currently is is very pale" If the new compounds' residual optical absorbances are in the green, the solution will be iodine purple by transmission.