Where and what type of coloring inks are used to add color to black/white photographs

where and what type of coloring inks are used to add color to black/white photographs
Dr. Ph. Martin's Dyes are water-soluable, very intense dyes originally manufactured for use on black and white photos. They're available at Blick and any other better art supply stores. They are dissolved in water to any lightness or darkness you wish. Since they are transparent, they won't cover up facial features or other photo details. I've used them for ages as a watercolor medium. There are several sets of colors available. They come in small bottles with eye-droppers because the colors are so concentrated. They may seem expensive, but they last forever because you dilute them. They are an truly awesome medium. Have fun with them!