The process engine status is in the Gray (Cannot determine engine status).
The process engine is not down if it is in the Gray status.
The process engine will typically go from Gray to Yellow (Running) status.
The process engine shows the current and past history of deployments. Green will never display as only colors to display are (gray,yellow,red).
Gray - Cannot Determine Status
Yellow - Running
Red - Stopped
So long as the process engine is not in the Red status, and is in the Gray or Yellow statuses, the process engine is up and running.
Another method to check if the process engine is active, is to check either the
Administration > Data Administration > Processes > Initiated or
Administration > Data Administration > Processes Engines > Events