Colour contrast

Sometimes you can give a picture impact by placing a light subject against a dark background. In fact, using contrasting brightness is a typical artist's trick. 

Here however I am using contrasting colours to draw the eye.

By comparison, let's try that same image in monochrome:

These splendid Autumnal leaves seemed to stand out agains the rare but welcome blue sky, and careful positioning means the picture contains just enough and no more.