HYDROINDIGO-90 is likely a commercial name for a form of leuco indigo, which is a water-soluble, reduced form of indigo dye. This form is crucial for the indigo dyeing process, particularly in denim production, as it allows the otherwise insoluble indigo to be applied to the fabric. Upon oxidation, the characteristic blue indigo color develops. The “90” might indicate the purity or strength of the leuco indigo product.