gistfile1.py

from kivy.app import App
from kivy.base import runTouchApp
from kivy.lang import Builder
from kivy.uix.widget import Widget
from kivy.properties import ListProperty
from kivy.core.window import Window

from colorsys import hls_to_rgb

class ColourApp(App):
    def build(self):
        root = Builder.load_string('''
<ColourWidget>:
    canvas:
        Color:
            rgb: root.colour
        Rectangle:
            pos: root.pos
            size: root.size

        ''')
        return ColourWidget()

class ColourWidget(Widget):
    colour = ListProperty([0, 0, 0])

    def __init__(self, **kwargs):
        super().__init__(**kwargs)
        Window.bind(mouse_pos=self.update_colour)

    def update_colour(self, instance, value):
        x, y = value

        x /= Window.size[0]
        y /= Window.size[1]

        self.colour = hls_to_rgb(x, y, 1)

ColourApp().run()