Rob, what Eric is saying is that the green color comes from the green plastic that the light shines through. You won't be able to change the color because it will still have a green hint to it from the plastic...even if you just added a colored sleeve to the bulb.
i would say to try a colord bulb in the back that would mix with the green to make the color that you wanted say like a red bulb would give you like a burgandy color not to many mix with green though but that is just an idea
The green-colored plastic is what makes the light come from all parts of the dash, it´s like poor mans fiber-optics. If you were to get a new color, you would have to remanufacture all the small plastic parts!