How can I change font color (red for deletions and blue for additions) with “Show Changes” turned on? It seems to get rid of all colored font.
(As far as I understand the question)
That’s really tricky, but the (modified/original) font color should reappear after “Accept or Reject Changes”.
(Related to → Trying to create a template with UK english, different font colors)
You can’t. The colors that you see will depend on your settings in Tools - Options - LibreOffice Writer - Changes. If you choose When you choose the condition “By author” in the list, the color is automatically determined by LibreOffice, then modified to match to the author of each change.
But these colors are temporary only. If you want to “preview” how the document will look like when you have accepted /rejected the change, you can toggle the “Show Changes” option. Or you can accept this particular change so that the color stays. But this will not be highlighted as a change anymore.