In terms of severity for each individual recipient the best outcome is Low; Low has the green color because from a business prospective this is the most desired outcome: user reads the email, identified it as phishing and avoided any dangerous actions.
Neutral - User never opened the email
Low - User opened email, but performed no action
Medium - User clicked the linked or replied to the email
High - User clicked the link and/or replied to email more than 1 in total (so either performed both actions or one of them twice or more)
Critical - User submitted a form with the password