The use of a preferential–solvation index (PSI) for the comparison of halochromic systems

•Halochromic systems analyzed with the aid of a preferential–solvation index (PSI).•The PSI approach is not based on any theoretical models, being derived from experimental plots.•PSI calculated for fifty examples of halochromic systems from the literature.•The observed trends interpreted in terms of probe–salt and solute–solvent interactions.•The PSI approach distinguished the behavior of organic and inorganic cations in solution.