Blue would be nice, though you'd need a mix of shades. Darker blues for the core outfits to allow pastel blues to be accents.
except that going on what information we have (her hair colour) and some
cursory research, Emma is likely to be a warm Autumn(1) which means pastel anything would make her look tired/drained at best, with her good options for blues in general being rather limited. c'mon guys, know your fashion basics
having said that, please remember that transformed appearance is not limited to what your regalia looks like, you can emphasise/modify her appearance in other ways as well.
Also remember that her transformed apearance should reflect her ideal self "unfettered by this world of sorrows.". we should be designing around who she wants to be, who she could be at her best; keeping in mind her defining characteristics(2), the nature of troubadours (3), her court (4), and to a much lesser extent (if at all) her invocations(5); (I don't like any of the costumes proposed so far as they say nothing about Emma, let alone about Emma's ideal self)
You stare at your new self. It looks like you, but at the same time, unfamiliar. No, that's not true, not entirely. This is the you that should have been, that should have been there at the alley, that should have found a way to beat back those gangers without having to see their corpses at your feet. This is the you that can take on the worst the world has to offer and win.
(1) the likely runner up is warm spring(which has more slack with the blue) but the kind of hair that's called 'red' without qualification is generally an autumn hair colour. winter (black, deep purple and other darker colours) and summer(pastel be here) are right out
(2) specifically the ones she likes about herself and those that are intrinsic to her image of herself / her ideal self. her heroes, role models and the people in her life that she looks up to would offer clues to this
(3) going by my understanding this covers anybody with a passion for there art/works and its sharing, with some edge case examples being things like Phantom/Magician Thief(6), Cosplaying Wonder, Master Bladesmith and World's Best Barista. what vocation is she called to and how will she express it.
(4) Courtless; so this one feeds back on herself; specifically her ideals, the issues she thinks are most important and what reason she has for fighting the darkness (in whatever form she chooses to fight it in)
(5) Freedom, Craftiness, and laughter for Aria; Determination, Drive and an unrelenting rage against the Darkness for storms, both have
countercultural symbolism as part of their theme and heraldry they also both have a tendency towards regalia that offer freedom of movement
(6) the kind that delights the audience with their daring performance as much as they frustrate the police
edit: for levinbolt; their is nothing that says it has to look like a practical weapon (their is even precedent for it not looking like a weapon at all), Emma has as much; if not more; reason to have an aversion to guns as to want one; she's neither inherently physical nor conventionally violent, and nothing about her character says that she associates guns with power or safety (you might have an argument for crossbows, but I don't think SS was using one in the alley); given that there is
absolutely no mechanical advantage to using a gun we might as well use needles (the ornamental kind you use in hair), or weighted scarf bolas, or aerodynamic gemstones; they're less likely to get us tasered by the police(or foamed by the PRT) for carrying around a weapon