I did a basic edit of the green plane to show you a few of the things i would do to improve that one.
The tail had shading from one side to the other gradient style,
And when you look at a metal pipe you see that it
has darker color on both sides.
notice how the pipe has darker color on both sides and the highlight is not
on the side side it's up a little.
another thing i like to do is after i have colored the entire thing i come back
and add shades that are in between the colors i used as a sorta color aa.
notice how every time i come to a edge in the shading i add a color that blends the edge into
the next shade.
Keeping the color count down is not important unless your trying to stick to a systems restrictions,
soo more colors can help smooth things out.