Tkinter, how to get a button's bg color

[upftftip at gmail.com]

I am new to Tkinter.  Following an example, I executed the following:

window = Tk()
b = Button(window)
b.configure(bg = '#FF00FF')
b.grid(row = 0, column = 0)

how can I later get the value of this button's background color?

Thanks.