Compute signal color probability (u_color) for each catalog object on the fly.

def calc_signal_color2(self, distance_modulus, mass_steps=1000):
        """
        Compute signal color probability (u_color) for each catalog object on the fly.
        """
        logger.info('Calculating signal color from MMD')

        mag_1, mag_2 = self.catalog.mag_1,self.catalog.mag_2
        lon, lat = self.catalog.lon,self.catalog.lat
        u_density = self.isochrone.pdf_mmd(lon,lat,mag_1,mag_2,distance_modulus,self.mask,self.delta_mag,mass_steps)

        #u_color = u_density * self.delta_mag**2
        u_color = u_density

        # ADW: Should calculate observable fraction here as well...

        return u_color