Making a window with a color box in RoboFont with vanilla

box-color-test.py

from vanilla import Window, Box
from defconAppKit.windows.baseWindow import BaseWindowController

from AppKit import NSColor, NSView, NSRectFill

class BigBox(BaseWindowController):

    def __init__(self):
        
        ## create window
        self.w = Window((720, 600), "color box", minSize=(72, 86))
        
        ## add a vanilla.Box
        self.w.bg = Box((10, 10, -10, -10))

        ## make the box red
        ###
        ## this part isn't working
        ## how should I be doing it?
        ##
        self.w.bg._nsObject.setBackgroundColor_(NSColor.redColor())
    
        self.w.open()

BigBox()