Observed and mean synthetic colours (J − K) vs. (V − K). The observational data (plus signs) is compiled from different sources: Galactic Bulge miras (Groenewegen & Blommaert, 2005, grey), field M-type LPVs (Mendoza, 1967, red), and C-rich giants (Bergeat et al., 2001, green). The model data (filled squares) is from models which develop a wind, as indicated in red in Fig. 1. All panels show the same data but with different colour coding according to stellar luminosity (upper left), effective temperature (upper right), seed particle abundance (lower left) and piston velocity (lower right). The stellar mass is one solar mass for all models. Most of the observational data are single epoch measurements, whereas the synthetic colours are means over the pulsation cycle.