You have to decide what your trees look like. What your sky looks like, what your grass looks like. You have to choose a color palette that will stay consistent and complimentary and your lines have to be believably the same all the way through!
Here is an example with birch trees. It took me 4 paintings (over 2 years) to discover the way I like painting birch trees. From now on my birch trees will look consistent. Now I need to apply that to everything else in my illustration world.