Photometric color of the program stars as function of their measured temperature (filled circles). The black curve indicates the fit of the data, Eqs. (3 and (4). The grey thick curve shows the theoretical zero-age HB of Cassisi et al. (2009). The same model, re-derived for the non-standard filters used in the adopted photometry, is shown with a dashed grey curve. Empty circles show the position of NGC 6752 stars (Moni Bidin et al. 2007).