Not completely certain of what you're referring to as the "background". Roboscreen captures the specified region and only what is captured is imported per se, so there is no default background colour. Perhaps a screenshot of what you're referring to might help.
Typical use for FM is to capture the portion of the screen, then save it as a file (use PNG if it includes text from the screen and isn't a photo). The saved file is then imported by reference into FM. The background colour is what is specified for the anchored frame on import.