Example py file. If I remove the Color line, the MyLabel will inherit the color of its enclosing Layout.

scrollview.py

import kivy
kivy.require('1.0.8')

from kivy.app import App
from kivy.uix.label import Label
from kivy.lang import Builder
from kivy.uix.floatlayout import FloatLayout
from kivy.graphics import Rectangle
from kivy.properties import StringProperty
from kivy.uix.image import Image
from kivy.graphics import Color

Builder.load_file('scrollview.kv')

class MyFloatLayout(FloatLayout):

    def __init__(self, name, *args, **kwargs):
        super(MyFloatLayout, self).__init__(*args, **kwargs)
        self.name = name


class MyLabel(Label):
    image = StringProperty()
    def __init__(self, *args, **kwargs):
        super(MyLabel, self).__init__(*args, **kwargs)
        with self.canvas.before:
            # If I delete the following line, this label inherits the Color value
            # of its containing Layout:
            Color(1.0, 1.0, 1.0)
            Rectangle(source=self.image, pos=self.pos, size=self.size)

        # listen to size and position changes
        self.bind(pos=self.update_rect, size=self.update_rect)

    def update_rect(instance, value, new_position):
        # value is a MyLabel object
        instance.rect.pos = instance.pos
        instance.rect.size = instance.size

class TestApp(App):

    def build(self):
        root = MyFloatLayout("root")
        odd_label = MyLabel(text="Oddball", image="disney.jpeg")
        odd_image = Image(source="disney.jpeg", size=[400,300], pos=[200,200],
                          size_hint_x=None, size_hint_y=None)
        root.add_widget(odd_label)
        root.add_widget(odd_image)

        return root

if __name__ == '__main__':

    TestApp().run()