My pal Margeaux Lucas has a blog, Paper/Pencil/Brush. She shows a lovely example of underpainting in a gouache illustration.
Her style reminds me of picture books I read when I was little. It’s all too easy to become heavy-handed with gouache, especially when you’re piling paint on top of paint, as you’re obliged to do with an underpainting. Margeaux has kept this enchanting little image light and fresh. Also, look how much information she gives you: time of year, place, who the main character is, anticipation of some future event—all important to an audience who is just learning to read.