You can add a custom field of type "Traffic-Light" on any request of JIRA ServiceDesk: so, you will have got that field on ServiceDesk Portal being visible for all logged-in customers as well. (Please refer to the add-on version release nodes as older version do not support this feature.)
This requires at least JIRA ServiceDesk version 3.4.0 or newer! Using oder software versions, your traffic-light custom fields will not be rendered smart: just the color name will be displayed on ServiceDesk Portal requests.
Configuration
Usage of Traffic-Light on JIRA Service Desk
While creating a new issue on JIRA Service Desk Portal, you can select any color of the configured traffic-light custom fields as shown below:
Viewing this request, you will see the content of the traffic-light custom field as colored image:
