Here's a (skintone) digital painting tutorial I made a while ago. My basic idea was to explain everything I know about observing, light and color in digital art using skin tones as an example. Each part of the tutorial contains an exercise. I hope it can be both helpful and fun!
(Disclaimer: this is just "my" way to learn/paint/observe, and I can't guarantee that it'll make things "click" for everybody; I'm not a teacher after all and still have SO much to learn!)
EDIT: Attached the image files using Conceptart attachment manager, since free hosting services aren't always so reliable. Hope it works better now!