It is well known that fluorescence line narrowed (FLN) difference spectra (DFLN spectra) reduce to singlesite fluorescence line shape function in the low-fluence limit. This finding is confirmed by the observation of different line shapes of FLN and DFLN spectra obtained for pyrene (Py) in ethanol glass at various excitation wavelengths using time-resolved fluorescence measurements. A peculiar dependence of the Huang-Rhys factor (S) on excitation wavelength was discovered, and discussed in terms of the solutesolvent, intermolecular Lennard-Jones potential. A V-shaped frequency dependence of the S-factor suggests that the repulsive-dispersive potential is not sufficient to account for a complex 'color effect'. (C) 2013 Elsevier B.V. All rights reserved.