[Frenzie]

https://www.epapyrus.jp/docs/djvuplu...g%20Layers.htm

In the DjVu format, a document is separated into a mask (or "foreground mask") and a background layer. The mask holds the sharp-edged elements such as text or distinct lines while the background contains fields of color with soft edges.

The mask is actually the black and white layer. Where does the color for the mask (i.e. for sharp elements and text) get stored? In two-layer documents, the mask layer has a palette of color information in it for rendering the mask in its proper hues. In three-layer documents, another separate layer called the foreground holds the color information for the mask.