In my experience, spotting pens are unlikely to be the best answer for anyone other than the folks who make spotting pens.

The best answer is to buy spotting dye in a bottle. Spotone is no longer made, but Marshall's has a line of dyes that is equivalent. You don't need all six colors unless you do a lot of exotic toning - start with the neutral dye.

My approach is to use some kind of pallette. I use a white plastic makeup container, but you can buy plastic pallettes at any art supply store. You will also need a spotting brush. A 4/0 artists brush is fine.

You first have to do a little preparatory work. For this Put a drop of dye on your pallette, and then smear it around with your brush to create areas of varying density on the pallette Let the dye smears dry. Put the bottle of dye away in a safe place (ie, where you can find it in a few years when you need to repeat the preparations).

Then, moisten your brush with a little water to which you've added some wetting agent. I have a small bottle of RO-filtered water (distilled water is OK) water that I've added a couple of drops of working-strength PhotoFloTo start on a print, choose a dye smear that is fairly light in tone. Pick up a bit of the dried dye on a barely-moistened brush. Using a stippling motion, apply a series of dye dots in to the area needing spotting. If the dye is too light, you can pick up more dye, or move to a smear that is darker in color.

Using a magnifier helps. I use high-diopter reading glasses from the drug store. It also helps to have a strong light positioned at an angle to the print. The most difficult part of the process is getting the brush into the right point in the spot on the print - a trick here is to look at the shadow of the brush cast on the print by the light. As you move the brush closer to the print, the tip of the brush will approach the tip of the shadow on the print. So simply bring the brush tip and its shadow together on the spot.

You don't have to make the spot match the surrounding area exactly - spots are intrusive because the eye automatically goes to bright areas in the print. So what you want to do is reduce the local contrast between the spot and the area that surrounds it so that it no longer appears to be bright.

A bottle of dye will last a very long time, especially if you use the dry approach that I've described. When you are done, put your pallette with its dried smears away for the next time you needs to spot a print. If you don't use your dried dye smears frequently, it is possible fhat you can get some mold growing on it (this may take several years to happen), If so, wash off the pallette and apply some new smears.