Seems as if he makes it a bit harder than it really is. I’ve used Testor’s paints in the past. Just fill in with a small brush or toothpick and wait a few minutes and then wipe off excess with a cloth soaked in thinner. Might take a few wipes to get it all and even if you can get everything off it’ll wear off eventually.
I’ve also used different colored crayons with some success. Just degrease area and scribble over area with crayon and then rub off excess wax.
Some people use fingernail polish but I’ve never tried that.
Experiment a little and see what works for you.