The Background Image zone consumes a video decoder on the BrightSign player. If the player model supports two video zones (e.g. XTx44, XDx34), it can support one Background Image zone and one video zone (or more with Mosaic mode), while single-video models (e.g. HDx24, LS424) do not support both a video zone and Background Image zone simultaneously.
If you only want a background color (instead of an image), do not add a Background Image zone. Instead, change the background screen color in the File > Presentation Properties > Main tab.