I just want to start by saying, next to colored pencils, watercolor is my favorite medium. If you want to use it in these coloring books, go for it!
I use watercolor for the background often AND I do my background first! I start by lightly brushing a little water on my paper, not too much or it will curl, even the best watercolor paper will curl with a lot of water. Then I drop color on by just barely touching my brush to the damp paper. This is called "wet on wet" and works well for making a mottled background, sky with clouds, background or even a sunset sky. This works great for a background behind plants and leaves. It's okay if you paint a little over the lines, you can color over it with colored pencils, pens or paint.
Once my watercolor background is dry, I work on the main part of the picture. I will often color the characters or main image with colored pencils, other times I use watercolor pencils again.
Using watercolor pencils are not only fun, but easy! They color just like a regular colored pencils, but then with a little water, they blend perfectly! For shading, color the area that you want the darkest, then with a wet brush, pull the color over into the area you want the lightest. Some watercolor pencils become more vibrant when water is added, others dry lighter. It's all trial and error, as with any part of the artistic journey, but that's part of the fun, too!
Whether using watercolor from a pan, tube or pencil, it all works wonderfully on the pages of Jane F. Hankins fine Art Coloring books. The paper they are printed on is AMAZING! Give it a try and post on the group page what you did and any thoughts. I'd love to know what you think!