Something I found helpful when I was learning how to approach things a little more scientifically was to shade the big shapes FIRST, then go back and start working on surface stuff. So for a tree, even if it had cool, gnarly detailing all over it, you would shade it like a big cylinder first. Then, when you could see how the light was reacting to the basic, major shape of the object, you can go back and start working in the details, using more appropriate colors (and with a better sense of the overall lighting).
BTW you are making great progress, keep at it!