I saw this done with chalk pastels, but I only have oil pastels, so I tried this out with them. I have smudged around with them before, but haven't really tried it out as a background in my sketchbook. What you do is take a cotton pad and rub it across your oil pastel. Then you take the pad and rub the colour on your paper with a bit of pressure. I have used my yellow oil pastel only, but of course you can mix different colours as well. If you have put a bit too much on the paper, you can lift some off with an eraser.
Then I drew over the background directly with ink. You can also draw on it with pencil, but if you erase the pencil later you'll also erase the oil pastel. Finally I coloured part of the drawing with coloured pencils. Some of them allow the background to shine through a bit, some don't. Of course, it also depends on how much pressure you use or how many layers you put on.
I think this is a nice way to make a coloured background.
