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.
Step 2: Add darker values and additional colors. For a strictly complementary underpainting, you will want to add a red with hints of blue to underpaint brighter, yellower grass areas, and a red with a hint of yellow underneath areas of darker, bluer greens.