Bottom panel: V −c_U,B,I diagram for NGC 3201 using photometry from Kravtsov et al. (2009) corrected by differential reddening but without decontamination from field stars. We selected a rectangle around the RGB stars and fitted a second-order polynomial shown by a dashed red line. This line is used to define δc_U,B,I as the difference between c_U,B,I and the fitted line for a given V magnitudefor the 28 stars analysed here. Symbols are the same as in Fig. 2. Average error bars are indicated at the upper right corner. Middle panel: correlation between δS(3839) and δc_U,B,I for all 28 stars. A linear fit to the S1 stars is shown by a red dashed line. Upper panel: same as middle panel but only for the 14 stars brighter than V < 14 mag. Average error bars are indicated at the upper left corner.