Hello Bittware,
SYST:PRES? will return 1 on success. It will power cycle the device, which will result in the LED turning off for a brief period. For most of our devices solid green indicates on and ready (but not active), while when actively making measurements you will see orange/red. Since preset will power cycle the device and return to power up conditions in the software, the device will be actively sweeping after preset and you will see red/orange LED.
I hope this helps.
Regards,
Andrew