I'm curious how that will turn out for you. I went to an art supply store and bought 2 sheets of professional color gels. It's that translucent color film they put in front of flood lights to change their color. What I don't like about the stock gauges is that they diffuse the numbers from behind with black dot matrix paint. I did try to scratch that stuff off but it's layered on there pretty good and I was afraid of dissolving the white paint that's on there.
Look through
My photo album (Link). It has some pictures that may help you.
My guess is that it will be verydark with blue, but with red it may be OK. Just keep in mind that the bulbs aren't putting out a true white light, and the orange comes through by darkening that color. Red and yellow make orange, so what you get may be a darker orange and not a true red. You could always go with LEDs, but I think you could learn some valuable info from your project. I haven't done enough experimenting with color gels yet. BTW my 25x40 inch sheets were 5.95US. Good luck, and let us know what you find.
Edit: Oh yeah, once you have the pod out, get some allegator click from radio shack ($5) and some lanttern batteries (or some 12v source or 6v in series) and test the lighting while it's out of your car. Trust me. experimenting this way is the only way to go so you don't have to keep going out to the car at night to see how it looks. My photo's show where to power up the backlighting.