WillemDafinky

the first part that clued me into this design choice was aang's conversation with tagah about restoring the airbenders. during the conversation, tagah's face is shadowed, and so is aang. tagah is successfully isolating aang here—as the last two airbenders left in the world, only tagah can understand aang. and aang, who is clearly so desperate for some connection to his people, is very much grabbing onto it.

the next time tagah and aang have a conversation with clear lighting/shadows, it's when tagah asks how aang can trust zuko after what the fire nation did to their people. when aang explains how zuko is a friend who helped end the war, aang's face is lit; tagah's isn't. i also find it really interesting that aang is leading the conversation here and placed on the right. in other scenes, including the one before this, tagah is usually placed on the right instead, and also usually is the one leading things.

and then we get into the immortal storm.

in the first shot on the left, tagah's face isn't really lit up or shadowed, but you can make an argument that some of aang is. either way, the important part here is that aang's face isn't in the shadows. then when tagah says that they can clear a path for the ship together, RIGHT after aang suggests that they find another path, the lighting flashes and immediately casts tagah's face in shadow.

(i'd need to rewatch to do a fact check, but it's important to note that, as far as i could see while looking for these specific moments to screenshot, a lot of the same lighting flashes usually illuminates the gaang's faces inside of the airship.)

but then the storm continues. and as aang is helping tagah, he starts to get more doubts about this plan of his. he turns around, face illuminated, and says they need to turn back as soon as he realizes they're falling behind.

but as soon as tagah convinces him that they need to push on forward and that his friends can handle it (which he admittedly isn't wrong) aang turns around and is IMMEDIATELY cast in shadows again. another important note is that tagah is STILL placed to the right/in the lead here.

as soon as they get through the storm, it's katara and aang's fight on the ship. when katara points out how risky what aang did was, her face illuminated. aang, who has already been convinced by tagah that he did the right thing, is cast into shadow still. (and tagah, like always, is off to the right and cast in shadows, too. the framing also puts him behind aang, almost like aang's reasoning isn't really his own here.)

when aang claims that none of his friends understand what he's been through—except for tagah, of course, even if it's never said—aang is still cast in shadow.

once they get successfully through the storm, aang tries to thank tagah for helping them navigate it. tagah insists it was all because of aang. both of them are still in the shadows. afterwards, when aang explains why they had push through, he still isn't fully illuminated. there's also an interesting detail of tagah literally putting himself between aang and the others (face still mostly in shadows) while putting aang on the right side. aang is the "leader" here, and DID believe in the strength of the team, but only after tagah told him to.

afterwards, aang is mostly illuminated during his search for the staff on the islands. i don't know if the lighting is ever as clear cut as it is during the flight/immortal storm scenes.

but at the end of the movie, once aang is safely on top of appa with his friends surrounding him after fighting tagah and falling through the sky?

he's back in the light—and alongside the real family he has left.