return colour to be used for rendering a flight mode background

def flightmode_colour(self, flightmode):
        '''return colour to be used for rendering a flight mode background'''
        if flightmode not in self.flightmode_colourmap:
            self.flightmode_colourmap[flightmode] = self.next_flightmode_colour()
        return self.flightmode_colourmap[flightmode]