Multi-colour validation example using simulated light curves. The figure shows joint posterior distribution of the true radius ratios and the host-contaminant effective temperature difference for seven scenarios. The simulations assume an M dwarf host star with T_eff = 3600 K and R_* = 0.4 R_⊙, a transiting object with an apparent radius of 3 R_⊕, three levels of contamination (0, 0.5, 0.93), and three host-contaminant temperature differences (0 K, 500 K, 1000 K). The c = 0 scenario corresponds to an uncontaminated object, while c = 0.5 corresponds to a scenario that would be expected if the contaminating star is similar to the host star (i.e. maximum amount of contamination that can be expected without significant colour difference), and the c = 0.93 scenario corresponds to a contaminated Jupiter-sized object. The dotted vertical lines show the true ΔT_eff, the dotted horizontal line shows the true radius ratio, and the solid horizontal line shows the radius ratio limit corresponding to our chosen 0.8 R_Jup brown dwarf radius limit.