Contrast

The idea of creating contrast is to attract the eye.

Now some of you might think that contrast is only a difference between white and black, but that's not all that I mean with contrast. Contrast is the difference between things. So contrast is light and dark, round and sharp, new and old, natural and technical, complex and simple.. You get the point.
You can also think that contrast is separating one thing from another, and you can take that thinking to very abstract level. For example paint something that hasn't been seen before and that way creating a contrast between existing paintings and your new painting.

" To have contrast, you need to have similarity. "

If you have a white paper that has one black spot in the middle, that black spot attracts the eye. Now if you fill the paper with black spots, then the eye doesn't attract to that place anymore.

Ways to attract attention

Just to name a few..

(Images are from national geography's photography contest)

Sharpness

Color

Size

Density

Things that don't belong

A familiar form

The danger of over doing
This is very common problem when creating contrast.
When you have too many of those red leaves that work as a contrasting element, the eye doesn't know anymore where to look.