With postfwd you could use the following rule:
id=RCPTCNT action=PREPEND X-RCPT-COUNT: $$recipient_count or something like that id=RCPTCNT01 recipient_count>=200 action=PREPEND X-RCPT-COUNT: RED id=RCPTCNT02 recipient_count>=100 action=PREPEND X-RCPT-COUNT: YELLOWPlease note that postfwd must be used at DATA or END-OF-MESSAGE stage to get recipient_count reliably.
cheers, jpk