The green colour is created by the printing on the applique (a thin plastic sheet with the graphics printed on it glued in to the cluster), the light shines through the applique and the printing filters the green light out from the white bulbs for night time but appears white by day.

So with out changing the applique it would not be possible and the aftermarket appliques are generally of poor quality when compared to the original item, believe it or not much money is spent in getting this illumination right.