Here's a portrait study of an imaginary person that I did as a demo for the digital painting class that I'm teaching. The focus was on color saturation and how that relates to color temperature. The excercise was to do a head study using just one hue and to vary the saturation of that hue to produce warm and cool colors. From that point, it's not much of a jump to go to full color with varied hues. The image above started out as the top left version in the image below.
I still remember the day I had the realization that the key to being able to paint was understanding the relativity of warm and cool. Once I started to figure that out, I started to feel like I knew what I was doing when painting. Well, sort of. So that's what I try to teach in my class: values, then warm\cool, then full color.