I did, in fact, use a photo filter on the blue displays (NOT the gauge faces. That's an 8 hour job.). It's a RoscoLux Medium Blue filter.
I had to fabricate a pointer puller to do this job right. You can throw off your calibration, cause your pointers to stick, cause them to rest below zero when powered on, and a few other issues that may, or may not crop up.
If you can pull the pointers without messing anything up, you will have one of two options (depending on the revision status of your cluster circuit card).
1. You can simply remove the metal bezel on the displays and insert the film.
2. You will need to unsolder the bezel from the circuit card and carefully pry it from the display.
Either way, be careful not to damage the glass display (if it is a glass type).
I'm not doing any mods, at this point, but will give advice as necessary.
Sorry I haven't been around, much. Thought I'd lend a hand, here