Fe1-xMgxMoO4 compounds with x = 0; 0.25, 0.5, 0.75, and 1.0 were obtained after annealing under inert gas at T = 700 degrees C. All of the compounds exhibit a pressure-induced and/or temperature induced phase transition between the two polymorphs adopted by AMoO(4) compounds (A = Mn, Fe; CO, and Ni). For the FeMoO4 compound, for both the alpha and the beta allotropic forms, the structural features have been correlated to the magnetic properties, the Mossbauer signals, and the optical absorption properties to gain a better understanding of the phenomena at the origin of the piezo(thermo)chromic behavior. The different contributions Of the Mossbauer signals were attributed to the different Fe2+ ions or Fe3+ ions from the structural data (Wyckoff positions, bond distances and angles) and were quantified. Furthermore, the low Fe3+ concentration (9 and 4 mol %, respectively, in the alpha and the beta allotropic forms) was also quantified based on the magnetic susceptibility measurements. The net increase in the Fe3+ quantity in the alpha-form in comparison the beta-form, which is associated with the occurrence of Fe-Mo Charge transfer, is at the origin of the important divergence of coloration of the two forms. To design new piezo(thermo)chromic oxides and to Control the pressure (temperature) this first order phase transition, FeMoO4-MgMoO4 solid solutions were synthesized. The optical contrast between the two allotropic forms Was increased due to magnesium incorporation, and the phase transition (beta -> alpha) pressure increased steadily with the Mg content. A new generation of nontoxic and chemically stable piezochromic compounds that ate sensible to various pressures was proposed.