Frame border color (fitting marker) depends also whether there are tags in an empty frame or not.

Four frames on the following screenshot have all the same format (Body) and are of the same type (Body).

1.The frame in the upper left corner is empty and not linked - it is blue. Please notice that format label is not shown if a frame is not linked and empty.

2.The frame in the upper right corner is empty and linked - it is red. Please notice that the numbering label is shown if a frame is linked and empty.

3.The frame in the lower left corner contains a paragraph style (not displayed on page) and it is not linked - but it appears is red because it is not empty, and it is not fitting.

4.The frame in the lower right corner contains a paragraph style and it is linked - it is red. Please notice that the numbering label is not shown if a frame is linked but not empty.