I have an altus azumi flute, made entirely of brittania silver with plated keys and a z cut embouchure. Right now I'm working on the chaminade concertino, and most recordings I've found flutist with a brighter tone seem to fit the piece more. My own tone is extremely warm and dark throughout until the extreme upper register, which I find strange because my flute is very light. I've compared my own flute to a friends powell auromite flute and it has a very bright sound. Normally I've been told that Japanese flutes where lighter while american flutes have a darker tone. Short of changing my flute (which I can't and won't do for years probably) is there something I can do to brighten my sound?

Sound production mostly depends on the player and the headjoint. If you can't see yourself purchasing another flute, try out different headjoints or adjust your embouchure. I own an American flute and a Muramatsu EX and I have to say that the Muramatsu sounds almost as powerful as my other flute.

You could try experimenting a bit with your embouchure. Then record yourself playing with the changes in your embouchure, and at the end of each recording, make a note what changes you made so you remember what changes you made. Make a few recordings experimenting different ways (rolling in/out your headjoint, lower lip out/upper lip out, more space between back teeth/vice versa, etc.). And after you've finished your dozen or so recordings, listen to them carefully and choose which adjustment you liked best. Then, play it for someone and see if they agree with you, especially your private teacher. Your private teacher probably knows your tonal characteristics quite well, so he or she could guide you better than someone online.

Another thing about the flute's natural sound is the shape of the tone-hole on the lip plate. Rounder tone holes have a brighter, more pure, golden sound, and squarer tone holes have a more dark, rich, silver sound. This is useful for predicting which flutes you will generally like the sound of more than others. I play on a Haynes (possibly one of the darkest sounding flutes on the market) so I tend to like darker sounds more. I personally think Chaminade has this haunting beauty to it when played on a darker sounding flute, and just light and simple beauty when played on a brighter sounding one. Copying recordings is useful for phrasing and musicality, but as far as a piece as expressive as Chaminade goes, creating your own unique sound makes it so much more beautiful than copying the sound of the person on the recording.

I am afraid I do not play the flute, so I can only comment on what it sounds like to listen to. The only recording that I have of the Chaminade (which may or may not be representative) sounds rather dull. I have no idea whether it is a dark or a bright sound. Probably dark. The thing the player needed to be able to do and either could not or would not is vary the tone and/or the volume. I suspect most non flautists would be like me and not know what a flute should sound like, but any musician will recognise a musical performance or a non musical performance. For example, the opening is marked mf dolce and letter A is the same tune but forte and finally 6 before C is ff. In my recording the 1st 2 are the same dynamic and tone and the 3rd is inaudible.
The other reason why I was dissapointed is that there was nothing that made me say "I really must buy a flute so that I can play this piece" but rather I felt I could just play it on my violin (or clarinet/ oboe /whatever) I am not sure what the solution is (or even if there is a solution) but this is what I would want to hear in a piece like this.