Your coloring is actually pretty good. Other then minor nitpicks, the only flaws in the piece are in interpreting the form of what your coloring. The only way to improve in that area is to strengthen your traditional art skills.
I am not sure about the claim that coloring someone else's lines will improve your own lines.
Coloring someone else's lines will improve your own coloring. Drawing your own lines is the only real way to improve your own lines.
