underpainting

noun2

1. •
   An initial layer of paint, often monochromatic, applied to a ground as a base for subsequent layers.

   • He would start with a stenciled acrylic underpainting and finish by making marks with the chalk and charcoal shells. — 2007
2. •
   A painting that the artist later painted over to create the final work.

   • At first they wouldn't believe him, but he gave them proof. X-ray the Adulteress, he told them, and you will find an underpainting. They did and found the underpainting van Meegeren had described. — 2012