def _find_colour(particle, start_index, screen_data):
        """
        Helper function to find an existing colour in the particle palette.
        """
        _, fg2, attr2, bg2 = screen_data
        index = start_index
        for i, colours in enumerate(particle.colours):
            if (fg2, attr2, bg2) == colours:
                index = i
                break
        return index