Background: A Tricky Composition

So, it's been a while. I have been slacking, not that anyone reads this anyway. But, for those that care, I will briefly discuss backgrounds.
Backgrounds can either help contribute to the composition or take away from it. The question is: What makes an effective background? An effective background (that isn't meant to lead the eye by orthogonal lines or have a usage with the focal point) should be relatively tame. That isn't exactly the artsy word for my thought, but, it should be a lot of things so that is the best way I could describe it. The picture I have included in this post was an experiment I did in an art class a couple years ago entitled "Excuse Me" (oils) that was featured in an art show. The idea I had was to use a potentially threatening background design to compliment the over-all composition of the piece. The design was an overhead view of city skyscrapers at night. The only reason it works is due to the color. Color is probably the most important move when creating an effective background layer. The colors I chose for this were taken from the palette that had the paint from the foreground and the objects in it. Though I used the same paints, they weren't nearly the same. These colors have been tinted, muted and are shaded variants of the original colors used. That means they flow with the rest of the piece but don't blend in.
One way to understand the color theory of backgrounds more easily is to look at the horizon line in mid-day. The sky directly over head is the most saturated blue, but, in contrast the horizon lines are much more desaturated. I won't delve into the science of it (mainly because I don't understand it completely, so I don't want to look like an idiot). Distance can be shown by the saturation of the background. With landscapes you can glaze a some cool light colors that seem to really push the distance. This theory comes from the vs. black idea. In a lot of cases black comes forward in a piece whereas white tends to fall back into the composition. Again,this is due to saturation. White is the absence of chroma/color and black (when dealing with pigment) is full saturation. Finding the right amount of saturation with the right colors is a whole other novel. So, along with sharpness, lack of detail and usage, consider the color itself. There have been many times when the background color has caused hours of pondering, but, hopefully this makes it a little easier for you.