Here's a little gouache in class demo I didn't get to finish in class - I'm still trying to figure out how to a finish in an hour. I'm just going to walk through the process a bit.
Here's the finished piece.
Hour 1: Working thin, and not using the full opacity of the gouache, I'm laying some basic warms and cools, focusing a bit more in the cools in the shadows. Im basically laying on some color and then subtracting out to maintain a little bit of a value structure. In all of these color in the face, I have a little bit of a pale coppery blue-green (besides the hair). I want that to be a unifying theme color.
Hour 3: Ok - so I like the warm, but now its too warm - so I bring back my tealy base color: a little darker, in the sky, and bit cooler on the ground. In the sky, I'm painting thick on top, going lighter on the bottom toward the horizon, so I can have a little dusky feel. I subtract out some secondary highlights in the hair, and add some darks in the hair. This is why I love gouache - you can still make some big changes, and blend.