Dark discoloration of an O/W cream with Metronidazole

It is known that certain raw materials as EMULG; Cetostearyl alcohol contain impurities. As the first products that are sensitive to that, I would designate salicylic acid and Sulpho resorcinol. Under the influence of iron ions, salicylic acid is purple-dark purple...... But salicylic acid comes in so many creams and discoloration is sometimes signalled. But here we also have metronidazole, which we cannot and cannot exclude. Therefore, a preview should be done   with a cream without metronidazole. Discolours that too? Although every product can have an influence, I now assume that sulfur does not play a role. If we want to exclude this then more splits have to be made.. Maybe postpone this again.