My original male is now 6 years old, and has browned a bit. However, he is still distinguishable from a normal. I have 4 Axanthic girls that I have produced that are laying this year, and they are from 2-4 years old. They are still very grey, although they do develop a bit of a purple tint as they approach three years old. I can get you some pics of my older ones if you would like. I would suggest you ask whoever you get your Axanthics from for pics of their adults, as almost every baby I have seen looks great, but some of the adults do "brown" alot. The parental stock is probably the best tell tale of how the babies will look.
Hope that helps,