Expected changes in colours and magnitudes of RGB stars caused by O-rich circumstellar dust shells for an optical depth at λ = 1   μm varying between 0 and 1, in the CMDs N_8.6   μm vs. (m_F606W − N_8.6   μm) (upper panel) and K_s vs. (K_s − N_8.6   μm) (lower panel). The different curves are for different dust compositions, namely the 100% AlOx (solid), 60% silicate + 40% AlOx (dotted), and 100% silicate (short-dashed) mixtures from Groenewegen (2006), and the silicates (long-dashed) from Bressan et al. (1998).