If you let Word assign colors "By author," Word ensures that different users
have different colors. You don't get control over which specific color is
assigned to a particular author.
If you are saying that Word indicates two authors in the same document,
check the user name (as Peter suggested). If you are using Word 365 (the
subscription version), you may find the following helpful: Click File tab |
Options; on the General tab, select "Always use these values regardless of
sign in to Office" (under "Personalize...").