(H, J − H) and (H, H  −  K_S) CMDs (top and bottom panel respectively) of the ONC, compared to the 1 Myr old isochrone model of DM98 (cyan line), the 2 Myr old isochrone of Chabrier et al. (2000) (green line) and the 2 Myr old isochrone of Siess et al. (2000) (green line). Synthetic photometry for these models has been computed using the grid of atmospheres provided by Allard et al. (2012) (in dashes) and then corrected using the corrections in Table 1 (solid lines), assuming R_V = 3.1. Dots mark the position of the spectral types listed in Table 2 along the isochrones (from K6 to M8.5 with increasing magnitude, from K6 to M6 for the model of SDF00). The dotted lines show the iso-density contours for the analyzed stellar sample.