The chemical structure you give has a subset of alternating single and double-bond (SDL) links. Molecules of this type have electronic resonance bands that may cause spectrally selective light absorption in the visible spectrum and when this happens a colour change is evident. The key factor is the size of the resonant structure. As its size increases the band migrates toward longer wavelength. Your observed change from yellow to purple is going in the opposite direction and indicates progressive destruction. The purple phase is thus down to initial size reduction in the length of the SDL chain and the terminal colourless stage follows the complete elimination of the relevant absorption bands.