Not that I have been able to do.
Typically you will see the use of a Calculation (with a Container Result) or an Auto-enter Container field that pulls colors from a utility Graphics table or something similar. (As one way to do it)
You can type a space on the layout and use the text object as a "background rectangle" with conditional formatting.
like proposed, you can use a text containing only a space...
but what I like to do is to create a field for this purpose inside my database, that is a text type global and contains only a space.
By using a field instead of just a label gives you the possibility to play with borders too. Then you can decide if you only want borders on the side and not on top, etc...