如何在Kivy上按下按钮时更改椭圆的颜色？

from kivy.app import App
from kivy.uix.button import Button
from kivy.uix.widget import Widget
from kivy.graphics import Color, Ellipse

class MyWidget(Widget):
    def __init__(self, **kwargs):
        super(MyWidget, self).__init__(**kwargs)
        self.color = (1, 0, 0, 1)  # 初始颜色为红色

        with self.canvas:
            self.ellipse = Ellipse(pos=(100, 100), size=(200, 100), color=self.color)

        button = Button(text='Change Color', pos=(100, 50), size=(200, 50))
        button.bind(on_press=self.change_color)
        self.add_widget(button)

    def change_color(self, instance):
        self.color = (0, 1, 0, 1)  # 更改颜色为绿色
        self.ellipse.color = self.color

class MyApp(App):
    def build(self):
        return MyWidget()

if __name__ == '__main__':
    MyApp().run()