Dereddened (W3−W4)_o vs. (K_s−W4)_o (top). Dereddened (W1−W4)_o vs. (K_s−W4)_o (bottom). The panels on the left show the models obtained with blackbodies of 3200 K and 2000 K and the astronomical silicate of Ossenkopf et al. (1992) and Suh (1999). The panels on the right show the models with the synthetic spectra of 3200 K and 2600 K of Allard et al. (2011) and the astronomical silicate of Ossenkopf et al. (1992) and Suh (1999). It appears that the curves of stars of 3200 K and 2600 K are similar and that the main parameter is the dust type and the maximum grain size. A smaller maximum grain size increases the span of colours (extending the curve to redder colours).