如何在Kivy标签上获得背景颜色？

from kivy.app import App
from kivy.uix.label import Label
from kivy.uix.boxlayout import BoxLayout
from kivy.graphics import Color, Rectangle

class MyLabel(Label):
    pass

class MyApp(App):
    def build(self):
        layout = BoxLayout(orientation='vertical')
        
        # 创建一个标签
        label = MyLabel(text='Hello World')
        
        # 设置标签的背景颜色
        with label.canvas.before:
            Color(1, 0, 0, 1)  # 设置为红色
            self.rect = Rectangle(pos=label.pos, size=label.size)
        
        # 监听标签的尺寸变化，更新背景矩形的位置和大小
        label.bind(pos=self.update_rect, size=self.update_rect)
        
        layout.add_widget(label)
        return layout
    
    def update_rect(self, instance, value):
        self.rect.pos = instance.pos
        self.rect.size = instance.size

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