Once a selection is made, make sure your right-hand selections panel is expanded.
Click the three-dot overflow menu to the right of where it says Boundaries.
Click Change Boundary Color — this will open a Boundary Colors modal.
From here you can select one of 8 System Colors that are in place, then hit Apply.
Or you can select the button next to Custom Color and enter your own Hex Code, then hit Apply.
Note: This is a universal change that will be applied to all of your boundary colors.