Red, Yellow, Blue
Green, Yellow, Purple
Purple, Red, Orange
Green, Orange, Purple
Red-Orange, Yellow-Green, Blue-Violet
You would shade it light, this way you don't take away from the objects.
You wouldn't shade it. The objects pop out enought.
You would shade it dark. You want high contrast. Light against dark is what you striving for.
You would shade it with a medium tone. It will give a minimal amount of contrast. That is all you need.
You don't shade the background that is agains the rules of drawing.