Hello! Several suppliers have various powdered and liquid colorants in various shades of red. To get a deeper or darker color you can also add some dark colored clays or even a little bit of Activated Charcoal powder. Bear in mind, if you go too deep in color you run the risk of turning your lather into "Psycho" shades of red or a horrible gray.
Sorry, I don't have any one specific colorant in mind - hopefully others will know of one or more for you.