The origin of the small scale color variations of the dust emission as revealed by the IRAS data in nearby interstellar clouds have remained a puzzle in spite of more than four years of dedicated efforts. One well settled result seems to be that these variations are due to local changes in the small dust particles properties (size distribution, optical properties). Spectral observations of the gas in a few bright mid-IR clouds (in the millimetric, visible and UV ranges) reveal that highly energetic processes are present in these clouds (excited transitions, broad lines, existence of molecules formed via endothermic reactions) although these clouds belong to the category of the "quiescent" ordinary interstellar clouds. The nature of the energetic processes at work in this component of the interstellar clouds is unknown. Magnetohydrodynamic shocks and intermittent dissipation of turbulence have been considered, but none of these processes is entirely satisfactory. Since we believe that this phenomenon is widespread in interstellar matter and may be of great significance for the evolution of dust particles and of that of the gas itself, we think that it deserves further investigation. The spectrometers aboard ISO are ideally suited for the spectral investigation of the properties of the highly excited gas, not only because the column densities of excited gas are small and therefore would escape detection by any less sensitive device, but also because the wavelength coverage of these instruments allows the observation of lines which will provide critical tests of the mechanism(s) which could be responsible for dust color variations.