I found this wonderful Youtube video of Wendy Hollender demonstrating her creative process. This is the way that I tend to work as well, with the toning of dark sepia to define the form and values of the subject. In this video, she uses watercolor pencil to cover the larger areas on the illustration. Combined with the FC polychromos undertones, this makes for a wonder mixed media piece.
For additional examples and instruction from these two wonderful illustrators, I highly recommend the following books:

Botanical Portraits with Colored Pencils by Ann Swan

Botanical Drawing in Color by Wendy Hollender
Before I began using these wonderful polychromos pencils, I worked through several of the exercises in Wendy's book and am presently revisiting Ann Swan's book.

(Here's a poorly lit photo of some exercises that I did from the Hollender book and a copy of a Redoute tulip that I had just toned with the dark sepia )