Potassium permanganate is beautifully violet, crystalline salt which is used in organic as well as inorganic synthesis as an oxidizing agent.
An azo dye for which I was unable to find any literature records. The data page contains absorption spectra in water at different pH, as well as timed decay measurements in acidic solution.
Obtained by reducing vanillin with sodium borohydride. The info sheet contains an UV absorption spectrum.