code: def calc_signal_color1(self, distance_modulus, mass_steps=10000):
          
          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
docs: Compute signal color probability (u_color) for each catalog object on the fly.