...
Click Settings > Issues on the top right hand corner.
Click Custom fields under Fields section on the left panel.
Creating the custom field
Click on the Add custom field button.
Click All or Advanced, search for RAG Status Field, select and click Next.
Fill in the necessary information, then click Create.
Select the screens to add field to and click Update.
...
On the View custom fields page, click Settings > Configure on the field that you wish to configure.
Under RAG options, click Edit RAG options.
Select the desired Display Type and Hide Text? (if you do not want the text Red / Amber / Green for the field) and click on Update.
Click Done to return to the previous page.
...