if you convert the file to an object , you can change the textures included in the object , I redomend you export from inventor in a diferent format than STL , because STL do not acept multi textures , its a geometry format only
or you can select faces to change texture , give click in the object and select faces with "Ctrl-Shift left Click mouse" , then left click in selected poligon and choose icon "custom face settings"
