ForeignLanguage = ColoredLabel(..., size = (a,b)) 
     
from kivy.app import App
    from kivy.uix.boxlayout import BoxLayout
    from kivy.uix.label import Label
    from kivy.properties import ListProperty
    from kivy.lang import Builder
    from kivy.core.window import Window


    kv = '''
    <ColoredLabel>:
        size: (self.size_x,self.size_y)
        pos: (0,0) # no effect
        background_color:
        canvas.before:
            Color:
                rgba: self.background_color
            Rectangle:
                pos: self.pos
                size: (self.size_x,self.size_y)
        '''

    Builder.load_string(kv)

    class ColoredLabel(Label):
        background_color = ListProperty((0,0,0,1))
        s_global = Window.size
        size_x = s_global[0]
        size_y = s_global[1]/3

    class MyWidget(BoxLayout):
        #init
        def __init__(self, **kwargs):
            super().__init__(**kwargs)
            ForeignLanguage = ColoredLabel(text="ForeignLanguage", size_hint=(None, None), background_color=(0/255,171/255,169/255, 1))
            Translation = ColoredLabel(text="Translation", size_hint=(None, None), background_color=(45/255,137/255,239/255, 1))
            Example = ColoredLabel(text="Example", size_hint=(None, None), background_color=(43/255,87/255,151/255, 1))
            verticalBox     = BoxLayout(orientation='vertical')
            verticalBox.add_widget(ForeignLanguage)
            verticalBox.add_widget(Translation)
            verticalBox.add_widget(Example)
            self.add_widget(verticalBox)

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

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