The Microsoft website has a detailed description on how to Use column formatting to customize SharePoint. In particular, it shows how to apply formatting based on the value in a field using a CSS class. Unfortunately, the article does not list the severity levels available. A little poking around at the style sheets uncovers the following severity codes:
- sp-field-severity–good
- sp-field-severity–low
- sp-field-severity–warning
- sp-field-severity–severeWarning
- sp-field-severity–blocked
Also of note, the example code for that section does not properly reflect the necessary code to reproduce the example.

"class": "=if(@currentField == 'Done', 'sp-field-severity--good', if(@currentField == 'In progress', 'sp-field-severity--low' ,if(@currentField == 'In review','sp-field-severity--warning', if(@currentField == 'Blocked','sp-field-severity--blocked', ''))"
}
If you read the JSON, the example overlooks the Has Issues condition. The JSON should actually read:
"class": "=if(@currentField == 'Done', 'sp-field-severity--good', if(@currentField == 'In progress', 'sp-field-severity--low' ,if(@currentField == 'In review','sp-field-severity--warning', if(@currentField == 'Has issues','sp-field-severity--severeWarning', if(@currentField == 'Blocked','sp-field-severity--blocked', ''))"
As well, the icon section leaves out Blocked. It should be:
"iconName": "=if(@currentField == 'Done','CheckMark', if(@currentField == 'In progress', 'Forward', if(@currentField == 'In review', 'Error', if(@currentField == 'Has issues','Warning', if(@currentField == 'Blocked','ErrorBadge','')"