I am using two analogous colours that should mix to create a colour that is found in between them. In this case I used cobalt blue and red violet with the expected result to be a blue violet. I wanted the final image to have a coherent colour. In other words, the different areas should achieve the same colour so that the final composition looks like a monochrome. Here are the steps that I took to do the demo drawing using three tone of cobalt blue, red violet and white.
For each layer of colour, I alternated between doing a red violet layer, then a blue layer.
A key element when doing oil pastel is to always work from lighter to darker and to use a medium to light pressure. As you can see from the steps above, the composition gets progressively darker as more layers of colour are added.
I hope the oil pastel enthusiasts out there will find this useful.