Model of stellar shell ejections represented here in a colour-colour diagram with wavelengths at 12 μm, 25 μm, and 160 μm, with two loops indicating the growing in time of the shells (see text). The red trajectory is for stellar mass losses of 10⁻⁷ M_⊙/yr and the blue is for 10⁻⁶ M_⊙/yr. Green filled triangles indicate observed colours of some giant stars together with their respective Li abundances. Both axes are represented by [λ₁ – λ₂] = log (λ₂F₁) – log (λ₁F₂). The observed fluxes (F)in Jy were obtained from Rebull et al. (2015) and AKARI and HERSCHEL catalogues.