Watercolor pencils are very similar to colored pencils in that, they allow you to have precise control, achieve great detail, they are very flexible, portable and inexpensive. The difference between regular colored pencils and water-soluble pencils are their leads. Colored pencil leads are either wax or oil based, whereas watercolor pencil leads have a water soluble base.
When working with watercolor pencils, it is important to remember that the results will not be the same as if you were working with colored pencils. After you have layered several colors down and have applied water or a solvent of some type, you will notice that the colors will sometimes be darker, sometimes more vivid, or sometimes will become a color you were not trying to achieve at all. So experiment before you start layering colors, to see if it is actually the color you want for your project or portrait. Also, try mixing your watercolor pencils with other mediums, such as Prismacolor colored pencils, Ohuhu Gel inks or Derwent graphite pencils. You will be surprised with the results!
I have discovered the following papers work the best when combining watercolor pencils with colored pencils: Arches watercolor paper, Strathmore watercolor pads and Rising Stonehenge drawing paper. Rising Stonehenge works best with the dry technique, with colorless blender markers, or with minimal water. If you do use water, make sure to tape the paper down to the board to prevent buckling.
All three brands of paper are acid-free and Strathmore also has ATC/ACEO size watercolor paper.
The watercolor pencil brand that I work with the most is, Derwent Watercolor Pencils. They are super creamy like Prismas color pencils and they have a wonderful selection of colors. They are fine art quality water-soluble pencil. They are non-toxic with lightfast pigment. They are available in packaged tins or open stock at most art stores.
In addition, I like to use Stabilo Aquacolor Watercolor Pencils to work on skies and small details. They have leads similar to Prismacolor Verathins, but, unfortunately, they have been discontinued. However, from time to time, you can find them on Ebay.
Keep on Creating!