LO can have underline color different from the text color. In presentation mode, the underline color is always the same as the text color, however.

Steps to reproduce:
1 Create new presentation. Empty Presentation, Screen, Default.
2 Click to edit title, enter any word.
3 Select the whole text or part of it. In Format>Character>Font Effects tab, select underline single, underline color Light Red.

Steps 2, 3 may be also done in the 'Click to edit Text' field, or in a text added by the text tool of the drawing toolbar.

In edit mode, the underline is shown in the correct color.

4 Switch to presentation mode (icon or F5)

Now the underline color is that of the text (black, if the text color has not been changed).  It should be red in the example above.

The bug was present already in previous LO versions.