Paints a cube at a certain position a color.

        Parameters
        ----------
        x: int
            Horizontal position of the upper left corner of the cube.
        y: int
            Vertical position of the upper left corner of the cube.

def paint_cube(self, x, y):
        """
        Paints a cube at a certain position a color.

        Parameters
        ----------
        x: int
            Horizontal position of the upper left corner of the cube.
        y: int
            Vertical position of the upper left corner of the cube.

        """
        # get the color
        color = self.next_color()
        # calculate the position
        cube_pos = [x, y, x + self.cube_size, y + self.cube_size]
        # draw the cube
        draw = ImageDraw.Draw(im=self.image)
        draw.rectangle(xy=cube_pos, fill=color)