Remove ALL manually overridden font size and font color settings
i.e. to get rid of all <p><span style="color:#000">text</span></p> and be left with <p>text</p>
What version of concrete5 are you using?
Are you referring to content added to pages using the Content block?
Using 5.6.31. Yes content block.
I wonder if you can search and replace in the database using regular expressions.