I did some testing of my layers for scrafitto work as Sian suggested:
This page below uses Caran d'Ache Neopastels on the bottom layer and oil pastels on top. I used various lino cutting tools to scratch. The lines here weren't too clear as the paper is quite textured and a solid colour from these oil pastels on top is harder to produce.
I was surprised to find that my oil pastels scraped off successfully from Koh-i-noor paints so my next uses bright Koh-i-noor paints on the bottom layer with dark oil pastels on top. Once the page was wet and painted, the oil pastels seemed to cover the surface more easily. I used a mini-screwdriver to remove the pastel as I needed a wider tool.