Spectro-photometric indices derived for our sample of carbon stars (brown symbols) and the grid of models (color symbols) as a function of (J−K_s). The triangles are for stars from Group A, the circles for Group B, the stars for Group, C and the squares for Group D. The filled symbols represent the averaged values of our indices in the groups, and the bars measure the dispersion within the bin. The open diamonds represent the models of solar (in blue) and subsolar metalliticity (red). The models were smoothed to R ≃ 8000 for the purpose of this figure. Typical uncertainties on individual measurements are shown in purple. The orange vector represents the extinction vector (computed for A_V = 5). In panel g), the green line was used to define the new index DIP153b (see Eq. (1)).