I've seen people with both
I never used either, but I saw a guy who used something to color his scalp (dermatch, perhaps), and a guy who used something to thinken his hair (topik, perhaps). Both had black hair, though the second guy had died it from blond. I spotted the first guy's cover up (one of my college instructors) because I noticed his hair line was higher in the center than on the corners, and thought that looked a little odd. I then focused in and could tell he had colored his scalp. The classroom lights were not very bright above him, so I don't know how he'd look in day light. I did not spot the topik in another guy's hair. It made his hair look thick, though he had plenty of thin hairs to put it on. It does not work if you don't have some hairs there. I knew about it because he showed it to me. He pinched his hair and pulled some of the powder off and showed it to me. So I guess it is not superglued on like they advertize. It is important to match your hair color to the stuff, and unless you use a good dye, most dyes are pretty obvious. I hope that helps.