Boolean logic to query tags?
Start a new find (command-f).
Under the main "Search" heading select the first pop-up and select "Other..." From the list of search terms, select "Tags".
In the second pop-up select "contains", and in the next field, enter a term. e.g. "Orange".
Over to the right, click the "+" button to search an additional term. In the new first pop-up, select "Tags". Select "is not" from the second pop-up, and enter the term e.g. "red".


This will select all files tagged "Orange", but not files tagged "Orange" and "Red"