Subsample size and spectral index. Blue asterisks mark selection according to Eq. (30), and red circles according to Eq. (31). The left panel shows the fraction of the Flagship catalog galaxies vs. β according to Eqs. (30) and (31). Symbol size increases from (I_E, Y_E), through (Y_E, J_E), to (J_E, H_E). The right panel shows the resultant α_ν vs. wavelength for the two subsamples following the notations in the left panel. The panel demonstrates the separability when Eq. (16) is applied at the marked Euclid bands of I_E, Y_E, J_E, and H_E.