If you ever want to recreate the marks you can make with pencils or pastels onto a painting then carbon papers are a great way to do that. You can buy ready made carbon papers from art shops but the colours are limited and I am not complete sure how light fast they are.
It is pretty easy to create your own with a little bit of denatured alcohol, your favourite powdered pigments and some waxed paper.
Using a jam jar mix up your powder pigment 50:50 with the denatured alcohol.
Use a brush to mix together.
And then paint the mixture onto the waxed side of your paper and leave to dry. The alcohol should evaporate fairly quickly and you will find that you should be able to make a number of sheets of carbon paper. I tend to do this in batches and keep the carbons in a plastic folder with sleeves.
With all the pigments that are out there you can create your own rainbow of coloured carbon papers!