I finished the underpainting today. The main idea of the underpainting is to get the drawing, colors, and major value shifts accurate in the painting. So for the stems I used a more planar approach with just flat colors, and for the peppers I rounded them as if they were generic cylinders. This way I have the basic color and values accurate for when I start to do my overlaying painting with oils which will make my life much easier.
I would move into working with oils now, but I should give this paint at least a day to dry.