Top panel: results for the simulation. Bottom panel: results for the field around LMC-ECL-09660. The distribution of (J − K) colour at the TRGB, estimated from the general (J − K)−T relation (the blue line in Fig. A.2) using an 0.5 mag interval below the TRGB is shown. The blue dashed and solid lines (roughly at half the maximum) indicate the mean and Gaussian dispersion, respectively. The narrower blue line above the peak indicates the formal error in the (J − K)@TRGB estimate. In the simulation the input was a Gaussian with mean 1.0 mag, and dispersion 0.05 mag. The analysis of the simulated data gives (J − K)@TRGB of 0.996  ±  0.025 mag, and a dispersion in the distribution of 0.063 mag.