Looking at it a bit more, I think controlling the brightness of the light source will be what makes the biggest difference. The way the current non-functioning bulb is mounted it points away from the meter.Depending on the colors of the meter, and the colors of the led, it might wash out certain colors. It's a cool idea! Be really easy to tell what mode your in that way!!
Depending on how you are switching the led, it might be possible to run 3 different leds that wouldn't interfere with the meter colors...
I ordered a big bag of cheap RGB LEDs to play with off of Amazon for under 10 bucks. Which will give me more than a few chances to screw this up and still be able to get the effect I'm after.