如何访问canvas中的颜色列表？Kivy

from kivy.app import App
from kivy.uix.widget import Widget
from kivy.graphics import Color, Rectangle

class MyWidget(Widget):
    def __init__(self, **kwargs):
        super(MyWidget, self).__init__(**kwargs)
        with self.canvas:
            Color(1, 0, 0, 1)  # 红色
            Rectangle(pos=(10, 10), size=(50, 50))
            Color(0, 1, 0, 1)  # 绿色
            Rectangle(pos=(70, 10), size=(50, 50))
            Color(0, 0, 1, 1)  # 蓝色
            Rectangle(pos=(130, 10), size=(50, 50))

    def get_colors(self):
        colors = []
        for instr in self.canvas.instructions:
            if isinstance(instr, Color):
                colors.append(instr.rgb)
        return colors

class MyApp(App):
    def build(self):
        widget = MyWidget()
        print("Colors in Canvas:", widget.get_colors())
        return widget

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