A much better and safer way
Get several different colored water based acrylic paints in those little squeeze bottles you can find at any local craft store. After the pups are born and fairly dry, you can mark each puppy with a different color.
Squeeze about a pea size drop of paint on to the back of the neck of the girls, and rump of the boys. Using a paint brush, dab the paint into the coat. Next, dab again to remove the excess with a paper towel being careful not to pull out too much of the paint. If you have a litter of 3, you only need to use one color. Say you used white paint, then you can have a white girl, white boy and a black girl/boy. Don’t worry if they smudge each other with a little paint, it will wear off the other puppies in a few days leaving only their mark visible.
The paint should stay on the puppies about 2-4 weeks before it begins to wear off to the point that it becomes hard to see any more, so you’ll need to repaint them again later on. I pick a time when Mom is out of the box, and all the pups are warn out and sleeping. That way, they are not getting paint on one another, and will allow the paint to dry.
I have found this method extremely safe for the puppies, it’s easier to identify individual puppies in pictures and I can tell who’s who at all times.