Simple colour-independent way to reproduce the ZAMS rotational distribution. Colour-period diagrams in GaiaG_BP − G_RP and B − V colours for the combined representative ZAMS distribution (data as in Fig. 17), showing how the elementary functions P = 45/τ_c and P = τ_c/8, of the convective turnover timescale τ_c may be used to specify its fast and slow rotation limits. The dashed line separates the fast and slow rotator regions in the Barnes & Kim (2010) model and approximately describes the less-densely occupied region between the fast- and slow rotator regions. This model is not relevant for the M dwarfs that are still on the pre-main sequence. See text in Sect. 7.3.