If my mostly black text document has some blue text throughout document that I would like to auto mass change from blue to green text.
If you have used a Character Style for the blue text then it is simply a matter of changing the Character Style and all text using that style will change, assuming no direct formatting
If you have used Direct Formatting rather than styles you might be able to change the colour using Find and Replace but it rather depends on consistent use of one colour of blue.
Identify the colour of the font that you are looking for from an example in your document and note the name or Hex code.
Click Edit > Find and Replace (Ctrl+H), in the dialog box that opens click once in the Find field then click the Format button near the bottom of the dialog. In the Search for formatting dialog click on the tab Font Effects. In the field labelled Font colour click the drop down arrow and select the colour that matches the one you identified. Click OK
Click once in the Replace field then click the Format button. In the Search for formatting dialog click on the tab Font Effects. In the field labelled Font colour click the drop down arrow and select the colour that you want as the replacement. Click OK.
You are returned to the Find and Replace dialog, click Replace All. Repeat for another shade of blue if inconsistent use of blue.
Ideally, in the Replace dialog you could replace with a character style but that doesn’t seem to be available yet.
1 Like
In addition to @EarnestAl.
Direct formatting often lead to inconsistencies in documents.
I recommend you learn how to apply style sheets:
Professional text composition with Writer and
[English documentation](https://documentation.libreoffice.org/en/english-documentation/)
An excellent solution! God bless you! 
I have noticed Replace All doesn’t consistently work and have had to replace one at a time, is this a known issue?
Certainly a drawback of direct formatting and not using a colour palette for the document.
LibreOffice uses a six digit hexadecimal number to define colours, that is 16,777,215 different colours. How many different shades of blue could there be in that range that you need to search for?
That is the beauty of using styles, you get the exact same colour everytime, you can change it once and every instance will change.
If you have to change every colour individually then you would be better to create a character or a paragraph style and apply it manually. Subsequent changes are then easier.