Have you ever wondered how to achieve a rich dark colour without using black? I don't think I own a single black pastel. But I do have a number of very dark colours in my pallet, and when combined, a rich deep darkness can be achieved.
This is the background of a painting I'm currently working on.
The top pastel is a dark blue.
The middle pastel is a dark brown.
The bottom pastel is dark purple.
I start with an under-painting.
In this piece I used a blue (slightly brighter than the dark pastel shown) for the under-painting.
By layering the dark blue, purple and brown I created a build-up of colour that offers depth to the background through varying strokes.
The dark blue recedes as the warmer brown comes forward. Creating interest in the empty spaces is as important as creating interest in the subject matter.
Peppers is a good example of the use of various dark colours to create an interesting background.