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Thymolphthalein is a colorant, or dye. We add dyes to products for a variety of reasons including helping you see where you applied the product, when a product is used up, or for aesthetic reasons. Thymolphthalein has a special purpose: It is an indicator dye that enables the product to spray on blue and change to white as it cleans. Thymolphthalein is a product of Milliken Chemical, which is responsible for its contents.