from guizero import App, Box, Picture, PushButton, Text



def change_colour():
    if mybutton.bg=="black":
        mybutton.bg="white"
    else:
        mybutton.bg="black"


app=App("a flashing button")

mybutton=PushButton(app, text="tick tock", command=change_colour)


mybutton.repeat(1000,change_colour)

app.display()