libreoffice massive text color change

Is there a way to change all occurrences of a specific color in a text in LibreOffice or Openoffice?

yes: With cursor in the find box:

• hit "more options" in the Search & Replace dialogue;
• click on "Format...";
• select "Font effects";
• select the color to search for;

Repeat the same for the replace box, selecting the desired new color (or "Automatic" to reset it to the default).