Upper panel: average Log versus B–V representing our 2D grid (orange stars). The lower solid line shows the basal flux used in this paper, from Mittag et al. (2013) and Schröder et al. (2012) and the upper solid line shows the upper bound for the activity level versus B–V, from Lockwood et al. (2007). The dashed line corresponds to the minimum Log we consider for stars with B–V above 0.94. The vertical dotted lines show the range in B–V covered by our simulations, the vertical dashed line vizualises B–V = 0.94, and the red horizontal line approximately corresponds to the Vaughan-Preston gap (see text). Lower panel: same for the S-index versus B–V.