Difference between colors of Pollux models with one parameter varied compared to the color of the reference model (red filled triangle). The parameters of the reference model are given in the upper panel, together with the values of the parameters that were varied. Colors based on Johnson photometry (HST WFC3 and ACS) are shown in the upper (lower) panel. C_X, =, (275–336)−(336−X), where numbers refer to magnitudes in a given filter (i.e., 275 is the magnitude in the F275W filter), and X is either the F410W or the F438W filter. Gray vertical bars indicate the typical separation between RGB populations in the cluster NGC 6752, according to Milone et al. (2013)