/ic/ - Artwork/Critique

Hi.
I'm new to digital painting, and painting in general, and I'm having a hell of a time to figure stuff up. I have doubts and I intend to explain part of my process in order to clarify why I have them. Thanks in advance for the help.

1 - Set Up
I usually paint in one layer with a round opaque brush. Seldom do I use an airbrush or a watercolor one. Every time I see a pro artist painting they use opacity brushes. Why? Even when they go to fill an area they use them instead of an opaque one. I can't get my head around it. Should I use more than one brush? Should I use more layers? Do I need textured brushes?

2 - Edges
Pic related is a work of mine that illustrates well my doubts. Focus on the gas pumps for instance. I paint an object with a color, them I shade it. At the end I paint the highlighted areas in the visible edges. I do it because I read that edges get the most of light. This holds true to everything else in the painting as you can see. Am I doing it right? Say I want to achieve a realistic look, why does it feels counter intuitive?

3 - Colors
How do people like Marco Bucci throw blue in a woman's face, or green in areas that usually shouldn't have these colros and make it look good? My colors in comparison look blend and boring. Instead of a realistic look, I get a cartoonish look.

4 - Overall painting direction / Style
How can I make my paintings look less digital and less comic-book like? I aim for a more realistic/fantasy style.

5 - Resources
Currently I have no resources in order to achieve my goals. What are some nice books I should be reading/creators I should be watching in order to improve my painting skills?