I don't think that any of the answers currently address this question. This question is referring to the new options to export a pdf with a watermark.
Export as PDF
General tab, see the
Check the box
Sign with Watermark and enter watermark text.
Text appears in the pdf in bright green.
Its not currently possible to change this value as its hard-coded into a patch which changed the color from bright red. This means that you'd have to change the value in the source code to do anything. The change was made to make the document more readable, and because the color is in-line with the LibreOffice colors. The discussion on the LO maillist is available here:
There is some talk there of addressing the issue, but i don't think anything has been done yet.