If you have enabled the "No Paint Warning" option under Settings - Robot settings - Alerts, you will get a notification when it is necessary to refill the paint container (or inspect the paint flow).
The tablet will emit a sound and display the message "No Paint!" even if the tablet is in sleep mode.
Image: Tablet screenshot showing the Low Paint Alert
Halt the robot and refill with new paint. Maneuver the robot back to the area it was in the process of painting, then press Resume, and the robot will continue from where it left off.