Nothing colours the tone more than the speakers that are outputting the sound. The theory behind a FRFR setup is that there is a very clean (typically) transistor amp that remains clean and doesn't distort or sag. The modeller is outputting the capture sound of a guitar rig, complete with the coloured output of the speakers of the guitar cab. If you are then putting coloured speakers on the end of your modelled chain, you aren't hearing the model as it's intended.
What you are saying is true - you can do this... but it won't be as authentic as a true FRFR setup.