Here's an explanation from an old thread. A YLFer provided this, and then Angie chimed in to confirm:
Angie described "false plain" in another thread when asked about her false black coat - I believe it just refers to a non-solid color that reads as a solid. For example, boucles or tweeds that are brown and white (or black and white, etc) can "read" solid brown at a distance. Solid fabrics can often show everything (hair, dirt, general schmutz), so having that variation of color can be a real benefit up close, as it will hide all those things.
