Maybe
It all depends on the design. If they are anything like others I have seen, the LEDs are soldered to the unit itself. You could remove the red LEDs and replace them with the purple variation, which would require hours of tedious soldering work. This might bring up power issues because I believe red draws the most power out of all LEDs and simply changing them to purple might burn out the new ones. Another option would require adding blue LEDs to other sockets if they are available.. then of course you might run into power issues, again depending on the design. Our boards have space for 3 LEDs. This allows us to use 1, 2, or Full color with a simple switch of the power device. Without seeing pics of the inside of the cabs, that's the closest i can get you to purple without replacing the display.
I do enjoy the idea of changing the face as it does seem the easiest way, didn't know that was an option for this job.