Iodine test is Specific to Starch -- To differentiate between Mono- & Di-saccharides (-ve), and Starch (+ve). The structure of starch is a such that the molecule is coiled. Iodine reagent (I2 + KI) forms I2KI, which interact specifically with these coiled molecules to form a blue colored complex. A positive iodine assay indicates the presence of starch, so if the color turns dark blue, blue or light blue, then this is a positive result.
