This page deals with slang terms for a Zombie apocalypse, in the heat of the moment it's easy to garble in horror at the situation and important messages are lost so easy terms are introduced by the military which are inevitably adopted and mutated by civilians.
In art, an underpainting is an initial layer of paint applied to a ground, which serves as a base for subsequent layers of paint. Underpaintings are often monochromatic and help to define colour values for later painting.
Building up layers of color with the oil pastel, called layering, is a very common technique. Other techniques include underpainting and scraping down or sgraffito. Turpentine, or similar liquids such as mineral spirits, are often used as a blending tool to create a wash effect similar to some watercolor paintings.