Normally its blue but you could also change it to yellow or purple.
Changing Your Color
To change your Boundary color, press the Oculus button on your right controller.
The menu will pop up.
choose the cogwheel on the far-right to get to your prefs.
There, on the left-hand side, you could find a section labeled Guardian.
The Guardian menu option
Here, scroll a little until you see the option labeled Boundary Color.
Its a drop-down menu where it’s possible for you to pick from three options.
Blue, yellow and purple.
Adjust this whenever you feel your boundary color doesnt suit what youre playing!
Its also where you’re able to draw a new Guardian Boundary if you gotta!