Hello,
There's no method to change all occurrences of a specific color in all targets: background, custom styles, default styles, contents... You need to use a set of methods: setBackgroundColor, setDocumentDefaultStyles, customizeWordContent.
If the color value to be changed is very specific and you are sure it won't appear in contents you don't want to replace, you can use rawSearchAndReplace that allows replacing specific text strings in all XML files of a DOCX.
Regards.