Placing a shape behind the text without putting the shape into the Section Master is possible only if the text is in a container (another shape or a text box).
In a Pages page layout document, each page is a separate section, all text is in containers, and the option to stack containers and other objects in any order is always present (provided there is more than one object to stack, and the objects are not currently Locked).
In a Pages word processing document, the main text is in the Document Body, and the only available space 'behind' the text is the Section Master space. Any object (including a colour-filled shape) placed in the Section master will appear (behind the text and behind any other objects not in the Section Master) on every page of the section.
Regards,
Barry