There is no scientific evidence to suggest that rosemary shampoo has the ability to darken hair. Hair color is primarily determined by the amount and type of melanin present in the hair follicles. While certain natural ingredients can enhance the appearance and health of hair, such as providing shine or improving scalp health, they do not possess the ability to alter the natural color of hair. If you are looking to darken your hair, it is best to explore hair dyes or colorants specifically formulated for that purpose.