Effect of the reconstruction error on ECS. Top: percentage residuals between mean BP/RP fluxes and corresponding forward modelled mean spectra computed from the ECS and instrument models as a function of wavelength; the plot uses data for the full set of SPSS, PVL, and NGSL samples. Bottom: residuals are normalised by the model error. In both cases, the colour map encodes the G_BP − G_RP colour index, while horizontal dashed lines placed at ±1 provide a reference. The vertical grey line at λ = 640 sets the separation between BP and RP data.