Correlations between intrinsec colours and between Q(λ) values and intrinsec colours. Left panel: Intrinsic (K_s−[D])o versus (H-K_s)o colours of Messineo et al. (2005), de-reddened using the median interstellar extinction of surrounding stars, shown in black. For comparison, in gray, we give the observed (K_s−[D]) versus (H-K_s) colours. Middle panel: Q(D) colours vs. (K_s−[D])o, for the MSX D-band. Right panel: Q(E) colours versus (K_s−[E])o, for the MSX E-band. The red dotted lines are fits to the data points. Error bars on the Q(D) and Q(E) are obtained by propagating the errors on the J, H, K_s, D, and E magnitudes.