The advantage of the crocein scarlet over an opaque dye is that the crocein is transparent so you can match the density of the negative around the pin hole. The idea is to use a dilute solution and slowly build up density until the spot matches the surrounding area. If you use too much the dye can be removed using a dilute ammonia solution. You can get 1 gram (a lifetime supply) for $18 from http://www.scbt.com/datasheet-214776...carlet-7b.html
You will also need a #000000 or #00000 round red sable brush. I would suggest practicing on a scrap negative. Remember that the red dye produces an actinic density that is different from the visual density. So make a few test prints until you get the hang of the techmnique.
See "Lootens on Photographic Printing and Enlarging" for how to mix up the dye solution.