work in shaping the observed luminosity and effective temperature distribution of stars along the HB is that there are many possible culprits (mass-loss, age, helium abundance ...; see Rood 1973 for example) and some of them are not well constrained from theory.\
Age has been identified as the natural global second parameter by many authors in the past years (Lee et al. 1987, 1988, 1990; Lee, Demarque & Zinn 1994; Sarajedini & King 1989). According to this interpretation older clusters tend to have bluer HBs, while younger ones should have on average redder HB morphologies. This scenario appeared in agreement with the picture for the Galaxy formation and its early evolution (Searle & Zinn 1978; Zinn 1985). By means of high resolution HST data for a large sample of GGCs, D10 found that the existence of outer halo GCs with anomalously red HBs fits well the scenario in which age is the second parameter. In fact, the behaviour of the 4-5 relatively younger clusters in their sample could be reproduced in term of correlation between age and HB morphology, while the bulk of the analyzed targets is peaked around old ages (see ages reported by Salaris & Weiss 2002, G10, 