In my case, the speedo, tach, etc, has a translucent white plastic with an orange coating (paint?) on the back side that filters out everything but the orange spectrum from the bulb. My plan is to remove this coating according to instructions on the net and use a different colored sheet of the gel to replace the stock orange with some shade of blue.

In some cases, the color does not come from multi-layered dyes and in some cases it does. What is sometimes done is to put a colored rubber sleeve over the bulb to filter and shine through white translucent plastic. Other times there are multi-layered dyes that impart the color you are seeing. I imagine if you are careful, you could remove the dyes down to the base white dye and use some small pieces of gel sheet or locate the rubber sleeves in your color choice. Excluding the gauges in my car, an Eclipse, everything orange is that way because of the rubber sleeves. I plan to remove those and place small pieces of the gel sheet behind the dash to make them light blue.

I'll post the results when I get around to doing it. I have some other things going on right now, and can't tackle that right now.

Stu