Colored pencils (This also works well with oil pencils)
Odorless Mineral Spirits (OMS)
Linseed Oil (or any vegetable oil)
Cotton swabs

Mix a small amount of the OMS with a few drops of oil.
Color image as desired with colored pencils.
Dip one end of a cotton swap into the OMS mixture and blot the excess on a paper towel. Use the moistened swab to blend the colors until the desired effect is achieved.

Start out with a very light touch and a well blotted swab. Once you see how the colors are blending you can be more liberal.