g′−r′ vs. u′−g′ color-color diagram of YSOs monitored with WWFI as gray dots. The bigger dots indicate the objects displaying accretion according to Manara et al. (2012). The gray cross shows the typical uncertainty of the photometry. The thick line is the sequence tracing the colors of the photosphere, obtained by synthetic photometry on spectra of diskless class III objects observed with X-shooter (Manara et al. 2013, 2017b). The colors of these template spectra are shown as green squares. The arrow traces the reddening vector for A_V = 1 mag, assuming the reddening law of Cardelli et al. (1989) and a galactic reddening parameter of R_V = 3.1.