Colour–colour diagrams for the O stars (blue), late-type stars (red) and early-type runaways (green). Left: 2MASS colour–colour diagram. We indicate the main-sequence with the solid grey line and highlight the position of an M0, A0, and B2 or earlier main sequence star with a grey label and dot. Reddening lines for an M0 V and a B2 V star are plotted assuming R_V = 3.1. Right: Gaia and 2MASS colour–colour diagram.