Top left: T_eff vs. V − J. The blue empty circles and green line are data from Casagrande et al. (2008) and Pecaut & Mamajek (2013), respectively. Black filled circles are the mean colours from 2700 K to 4600 K in our sample, with a size proportional to the number of stars. Top right: r′−i′ vs. spectral type. Open circles are the mean colours of Hawley et al. (2002, blue), Bochanski et al. (2007, “inactive” colours, green), and West et al. (2008, red). Black filled circles are the mean colours from K5 V to L2 in our sample, taken from Table A.2, with a size proportional to the number of stars. The error bars are the standard deviation (Hawley et al. 2002; Bochanski et al. 2007) and the intrinsic scatter of the stellar locus (West et al. 2008). Bottom left: g′−r′ vs. r′−i′. Black filled circles are the mean colours from K5 V to L2 in our sample, taken from Table A.2, with a size proportional to the number of stars. Blue empty circles are the mean colours by Davenport et al. (2014), with a size proportional to the numbers of stars, and the green line links the mean “inactive” colours by Bochanski et al. (2007) for spectral types M0 V to L0. Bottom right: M_J vs. J − K_s. Green and blue empty circles are data from Knapp et al. (2004) and Lépine et al. (2013), respectively. In the four panels, the stars in our sample are colour-coded by spectral type, and the discarded stars are plotted with grey empty circles.