I'm doing a new Plant Delights Nursery catalog cover this week. When starting the finished art, I like to do one area a little more finished, like the entire cover will be, to set the tone for where I'm going. This portion shows my sepia line sketch, lightened, so I can paint right over it.
I like this approach better than drawing the finished line work right away, because it helps the illustration look more like a watercolor painting rather than a line drawing that has been colored.
For the rest of the illustration, I'll apply light washes overall and gradually build the entire scene up to the level of finish of the completed face. Ideally, that will require less line work as a lot of the outlines will be defined by the washes.