How to update the RAG Status via Automation for Jira
Overview
Automation for Jira only display the custom fields with built-in field types for selection.

This page teaches you how to update RAG custom fields using advanced field editing method which can be easy too.
This method can be used for the following automation actions:
Steps
Click on the More options to expand the hidden panel
Paste and update the JSON string below into the Additional fields using either approach
By Custom Field ID (recommended)
CODE{ "fields": { "customfield_XXXXX": {"value": "Red"} } }
By Custom Field Name
CODE{ "fields": { "RAG Status": {"value": "Red"} } }

You can find out the custom field ID with the steps from this KB.
Possible values to set are
Red
Amber
Green
Demonstration
The video shows an example on how to change the the RAG Status field to RED automatically when issue is overdue using Automation for Jira