I share your pain (hate it when snakes look awesome as babies then turn into drab adults)... but all snakes do that, at least every species I have ever kept (well, I guess except for rat snakes [Elaphe guttata], especially Everglades rat snakes - they turn from drab babies into anyone's guess, but potentially stunning adults).
The best thing you can do to learn to what degree the different axanthic lines brown out (because they all almost certainly do) is to talk to a breeder and look at some full-grown adult breeders - especially the parents of any babies you may be thinking of purchasing. I have always heard great things about the VPI line axanthics, but I have yet to see any good pictures of adult axanthics of any line!
Good luck.
~Rebecca
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1.0.0 Dumeril's Boa '04
1.1.1 Ball Pythons
[1.0.0 '05 Orange Hypo (Specter)]
[0.1.0 '05 Het Orange Hypo (Sylvia)]
[0.0.1 '03 Normal (Sue)]
0.2 American Pit Bull Terriers (40lb darling lap dogs:Brandy&Mara)