Application of our technique to estimate the relative ages of MPs in NGC 104 and NGC 6121. In the observed m_F814W vs. m_F606W − m_F814W CMDs POPa (green), POPb (magenta), and their respective MSTOs are plotted. The inserts (a) show a zoomed of the MSTO region. The blue and red fiducial lines are representative of POPa and POPb, respectively. The theoretical models in the insert (b) were obtained assuming the parameters list on the top of the figure.