Instead of changing the color, add a new color bulb to the scheme that will change the color display. For instance if you have red now, add blue and you will end up with a green. You can probably make this happen by adding some neon tubes behind the cluster and a separate switch toa ctivate them.
But, if you want light green all the time, pull the cluster out and replace all the bulbs. There is no law that I am aware of that requires the dash lights to be any particular color - just visible at night.