Wavelength-dependent errors between ECLIPSE-Xλ and the R-TLSE (Eq. (6)) for LHS 1140 b, computed using a fixed filling factor (f_het = 0.15) and a temperature contrast (ΔT) varying from −300 K (spots) to +300 K (faculae). The largest discrepancies arise in the optical range (500–800 nm), where steep limb-darkening gradients drive optical biases. The dashed lines mark reference levels at −50, −20, −10, 10, 20, and 50 ppm, serving as a visual aid to highlight variations in the data. By construction, positive values of the error indicate that R-TLSE predicts a shallower transit depth than ECLIPSE-Xλ (overcorrecting stellar contamination), while negative values correspond to a deeper R-TLSE transit (under-correcting contamination).