A cream with salicylic acid is purple after an hour. Salicylic acid 5g Betamethasone diproprionate 0.05 g Tween 80 5g Ung aquosum Aqua AD 100g

A purple discoloration of salicylic acid indicates the presence of Fe ions. Therefore, I suggest to add the cream either 0.1% citric acid either 1% Na2EDTA.