What artistic elements to add to a car to diminish a difference in color shade?
Add high contrast polka-dots all over the car. This will totally upstage the hand painted areas and make the fact that they don't match moot. It could be any other pattern, like zig-zagging lines, but polka-dots are the most effective due to the euphoria polka-dots trigger in the human mind.

BTW - You will never be able to just cheaply match an original or pro paint job. Auto paint is a multi-layered process involving wet sanding the layers, applying a clear top coat, etc... with specialty paints. With metallic (or flake) paint that gets even more involved. Pro painters often brag about the number of layers of top coat they use in metallic finishes to get "depth" to the effect. And even then, using the same color will not match because of fading, even if slight, of the original paint.