Don’t be afraid if your Onewheel GT flashes green during a boot or power of this is normal behaviour.
Any time a Onewheel is powered on or off, the board goes through a system check, part of this includes the Lightbar. On the GT the system briefly flashes green on the right side when it is checked and will turn off. This blink does not constitute an error. Here is a complete list of the error codes of the Onewheel.
If your board does experience an error the lightbar will flash yellow or red a number of times and the number correspond to the error code. Count the number of times and look up the error in the error list.