Book series that had tiered magic, corresponding to colors of the rainbow [duplicate]

Question

I have a really hazy memory of a fantasy book series for children/teenagers that I read in the early/mid 2000s (in German, but I don't know if it was translated from English). It might have been born out of the Harry Potter craze.

It involved a sort of tiered magical tower or city (?) where each "level" corresponded to a color from the rainbow. "Red" inhabitants were basically living in slums and had barely any magic available to them, while the "Indigo" people at the top were the powerful elite (both political and magical). There were 7 castes/colors in total (as there are 7 colors in the rainbow).

The story involved a boy and a girl that somehow ended up in a higher level than they were allowed to. They found out, I think, that the ability to cast higher colored magic was not something you inherently were gifted with, but it was something you could learn. This was kept a secret by the powerful upper castes, to prevent an uprising (?) from the more numerous lower castes.

I borrowed these books from a library back then, so I think they would have been reasonably famous at the time, but it's been almost 20 years.

Answer 1

This could also be Garth Nix's The Seventh Tower (warning, TV Tropes entry) series.

A six-book fantasy series by Garth Nix concerning two groups of people living in a world of complete darkness, thanks to the Veil, a powerful black shadow that envelops the entire planet and blocks out the sun: the Chosen, a group of light-manipulating magicians who live in a multicolored Castle and believe themselves to be superior to everybody, and the Icecarls, a warrior race eking out an existence on the endless Ice beneath the Castle. The two do not mix (indeed, the Chosen do not even realize the Icecarls exist), but are thrust together when an ancient war between the the people of the Dark World and the spirit realm of Aenir threatens to flare up once again.

The books follow Tal Graile-Rerem, a young boy of the Orange Order of the Chosen, and Milla of the Far-Raiders, an Icecarl girl in training to become a Shield Maiden, as circumstances force them to rely on and trust one another and they attempt to unravel the ancient secrets surrounding the Castle and the war against Aenir, so long ago.

Among the tropes:

• Color-Coded Castes: The Chosen's society is arranged in a Rainbow Motif to match their light-based magic, determining the colours they wear and the privileges they're allowed. There is some caste mobility, so everyone dreams of ascending to a Violet lord, while the Reds are one misstep away from losing their magic rights and being demoted to white-robed Underfolk.
• Color-Coded Wizardry: The Chosen society use their Sunstones (capable of generating light) for just about everything, ranging from throwing around rays of destructive light in battle, to creating Hard Light structures, to healing wounds, to simple etiquette and courtesy (woe betide anyone who is bathed in the White Ray of Disgust, and does not give the Blue Ray of Humble Apology in return).

The hierarchy of the magic colors matches your memory. I haven't read the series, but poking around, Tal finds the Purple Keystone, which gives him the right to be emperor, which suggests that the colors of the sunstones that the Chosen use have more to do with what magic they can do than what caste they are born into.

If this is the answer, it is a dupe of Female main character in an ice world earning gems/runes

Answer 2

It's not a direct match, but could this be The Wind Singer by William Nicholson, first book of the The Wind Singer trilogy?

The book begins in the walled city of Aramanth, an extreme meritocracy where endless exams and ratings are the only way to move forward to improved life stations; to be unsuccessful in this is seen as a great source of shame. Using a system based on colour classifications, the governing Examiners dictate what people can wear, where they can live and what jobs they can do. The levels are grey, maroon, orange, scarlet and white, with the muddy Underlake the lowest and white the highest. The Emperor is the only person allowed to wear blue.

A minority in their society, the Haths believe more in ideas and dreams than in endless toil and ratings. When young Kestrel defies the harsh classification system of Aramanth she flees, finding herself in the company of the Emperor of Aramanth. Thought to be the ruler of the city, he is found to be merely a puppet of the High Examiner, and the Emperor tells Kestrel of the need to rid Aramanth of the influence of the evil Morah, of the need to return the voice to the mysterious Wind Singer that stands in the city arena.

Using an archaic map given to her by the Emperor she sets off, joined by her twin brother, Bowman, and their brave but pitiful new friend, Mumpo, who has an unshakeable affection for Kestrel. They meet a variety of tribes and individuals including the fearsome nomadic clans of Ombaraka and Omchaka. The journey eventually leads them to the Halls of the Morah; the very heart of the evil that has taken control of the city. Here the children finally retrieve the voice of the Wind Singer, in the process waking the terrible Zars, an army of the Morah. Pursued by the beautiful, evil and unstoppable Zars, the children race back to Aramanth, arriving just in time to return the Wind Singer's voice. The voice allows the Wind Singer to emit a powerful song that destroys the Zars and saves Aramanth.

One thing that may not match is, well, magic. It seems that the magic is entirely related to the "Singer" although the protagonists start gaining more access to it in the subsequent books.