How do I change the color of a rectangle I have already drawn in Python3 Kivy?

I'm trying to understand how to use Kivy - specifically, the canvas. I have more or less understood how to move around and resize a rectangle I've already drawn, but I can't get the rectangle to change color. Here is my code:

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


class CanvasWidget(Widget):

    def __init__(self, **kwargs):
        super(CanvasWidget, self).__init__(**kwargs)
        self.click_occured = False
        with self.canvas:
            Color(0, 1, 0)
            self.background = Rectangle(pos=self.pos, size=self.size)
            Color(1, 1, 1)
            self.rect1 = Rectangle(pos=self.pos, size=(self.width/2, self.height/2))
            Color(0, 0, 1)
            self.rect2 = Rectangle(pos=self.pos, size=(self.width/4, self.height/4))
            self.bind(pos = self.update_rect, size = self.update_rect)

    def update_rect(self, *args):
        self.background.pos = self.pos
        self.background.size = self.size
        if not self.click_occured:
            self.rect1.pos = self.pos
        else:
            self.rect1.pos = self.x + self.width/8, self.y + self.height/8
        self.rect1.size = (self.width/2, self.height/2)
        self.rect2.pos = self.pos
        self.rect2.size = (self.width/4, self.height/4)

    def on_touch_down(self, touch):
        self.click_occured = True
        self.rect1.pos = self.x + self.width/8, self.y + self.height/8
        #also, change rect1's color from white to red


class CanvasApp(App):
    def build(self):
        return CanvasWidget()
CanvasApp().run()

In place of the comment at the end of the on_touch_down method, I would like to change rect1's color to red. It has to be a change to the existing rectangle and not the creation of a new one since I want it to remain behind rect2 and I don't want to manually redraw the latter. If possible, I would like to achieve that without using the kv language since I want to be able to create rectangles, labels etc. on the fly while the code is running, and that seems to be inconvenient with the kv language.

An even simpler way without using groups is to just modify the rgb of the Color. First save a reference to the Color in your __init__() method by replacing:

Color(1, 1, 1)

with:

self.color1 = Color(1, 1, 1)

Then, in your on_touch_down() method, add the line:

    self.color1.rgb = (1, 0, 0)

See the documentation.