Ok,
I got prothik today and tried it out. This comes in a spray can. This compares with toppik (a dry powder) which can either be sprinkled on with a "pepper shaker" lid or with the optional sprayer attachment. Dermmatch is more like makeup. It comes in a shallow tin can (like shoe polish) and you spread it on with the supplied applicators (like small foam paint brushes).
Application of Dermmatch is the easiest for me to control and fine tune. It also stays put after it dries. It not only colors the scalp to remove the scalp/hair contrast, but also coats and thickens the hair. The other two tend to smear, spread, leave comb tracks on your scalp and forehead.
I have a hard time applying all 3 to the vertex of my scalp. To see what I am doing I need to use a hand mirror reflecting off the wall mirror. I can't figure out what is right/left and up/down while I'm doing it and I tend to make a mess.
My guess is that if you can coordinate your hand motion looking in 2 mirrors, the toppik and prothik would be useful for thinning at the top/back of the scalp; but not the hairline.
Even with dermmatch, you have to be careful around the hairline, but at least up front I'm only looking in one mirror, and can coordinate my movements and I can vary the density of the application to get natural, undetectable coverage.
I haven't tried couvre yet. It sounds similar to dermmatch, but I'm satisfied with the Dermmatch so I'm going to stick with that.
Good luck.