I tried Marshal oils and really didn't like the look of them at all. What I really like is using colored pencils - mainly prisma colors, and use them with the two solutions that Marshal makes (P.M. Solution and Marlene). The solutions allow you to blend the colors easily because it tends to disolve the colors and lets you work with them better. Most of my hand coloring is only spot areas in a print, not the full print.
Prints should be on matt or semi matt surface (I use Ilford MGFB warmtone). Use a fine brush to apply the P.M. Solution onto the area to be worked on and let it soak in a little - maybe a minute at the most. Then just draw on the image with your pencils and work them in together allowing the solution to do it's thing. You can use many different colors at a time. You have to work quickly and only coat a small part with the solution at a time because it starts to soak into the print after a few minutes. When your coloring is complete, let the image "cure" for a little while. Then use the Marlene solution to remove any of the P. M. Solution that has spread outside the area you have worked on.
Note that the P. M. solution is an oil based product and the Marlene solution is a solvent. What you are doing is applying the oil based product to the print to allow it to take the colored pencil better then using the solvent to clean up the edges. This is needed because the oil based product spreads out a little after it starts soaking into the paper. All you really need are some Q-tips and a tooth pick with a little cotton from a cotton ball rolled/spun onto it's tip. The Q-tips will do most of your clean up and the tooth pick is for really tiny areas.
Here is one of my hand colored images. Print was sepia toned a little first.