Validation of spectral parameters for low-frequency foregrounds, thermal dust, and CO emission, evaluated from the FFP6 simulation. Each histogram shows the error distribution at the two leading sub-dominant frequencies in the form of the normalized errors δ = (A_out(ν) − A_in(ν)) /σ_out(ν) for all pixels outside the Commander likelihood analysis mask, where A_out(ν) is the predicted foreground amplitude at frequency ν given the estimated amplitude and spectral parameters, and σ_out(ν) is the corresponding standard deviation computed over the sample set.