Marshall's is the retouching dye company. I think you can find them by asking the computer to find Marshall's Photo Dyes. I believe Michael's also carries Marshall's dyes, but only in the black/sepia tones, not the colors.
I've used them for years. My very first job I ever had, in high school, was photo retouching for a lg. studio that did school photos, fire dept, police dept, etc. I still have the brush I used, about 3 hairs, and the original bottles of the dyes. They last a long time if you keep them sealed, and then if they do dry up, you just add water.
The retouching dyes, come in oil, for "colorizing" photos. I wouldn't use that, but the "ink" dyes are for regular photos, touching up spots, etc. Practice, practice practice on your own things before you try a customer's, or pass it on to someone who's qualified. The latter might be my recommendation, but if Diane is an artist, she might have the "touch". It requires a light hand and good eyes, you dot-dot-dot, as in putting pixels and you have to look very closely. A green area might require dotting with red and yellow and blue inks. It isn't as easy as you might think.I'm probably the only person in town who does this, and get referrals fom the photo studios, etc. If you can master it, it's rewarding. Many things can be done via computer, but once in awhile it can't, as in the antique convex oval photos, or just a few minor spots that don't warrant the expense of computering.
Practice, practice, practice.