def calc_signal_color1(self, distance_modulus, mass_steps=10000):
        """
        Compute signal color probability (u_color) for each catalog object on the fly.
        """
        mag_1, mag_2 = self.catalog.mag_1,self.catalog.mag_2
        mag_err_1, mag_err_2 = self.catalog.mag_err_1,self.catalog.mag_err_2
        u_density = self.isochrone.pdf(mag_1,mag_2,mag_err_1,mag_err_2,distance_modulus,self.delta_mag,mass_steps)

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

        return u_color