The querent later identified in their question that this was the The Seventh Tower series by Garth Nix, a six book series that was the result of a joint partnership between Scholastic and LucasFilm.
Tal is a Chosen from the Castle, and Milla is an Icecarl from the Ice. Together they discover that an evil, long thought to have vanished, once more threatens their world, slowly and secretly letting its presence be felt. A pact was broken, and a war dormant for two thousand years is rekindling. Danger looms, and it is up to these two children and a small band of unlikely heroes to save their Dark World.
A thirteen-year-old Chosen boy residing in the Orange level of the castle, Tal is described as having scraggly hair and shorter than most boys his age. During the beginning of the series, he is said to form a curl when he smiles. Tal is bashful about this, believing it makes him look slow (which in truth, no one else believes so). His father is Rerem, guardian of the Orange Keystone; his mother is Graile; his little brother and sister are Gref and Kusi respectively. His aunts, Lallek and Korrek, both hate him rather exceptionally. He has an eccentric great-uncle named Ebbitt whom he is fond of and usually plays games of Beastmaker with.
Milla was a princess from the town close to the fear and close to Tal's age belonging to the clan of the Far-Raiders. While out in the Ice driving her sleigh, she discovers Tal, who she believes is a thief. Her attempts to kill him was stopped by her Crone. She knocks Tal unconscious instead and she grudgingly helps bring him back to the Far-Raiders' ship. There, she makes many attempts to fight him, including provoking Tal numerous times. When the Crones hear Tal's story, they decide that he is not a thief, and they will allow him to return to his Castle. However, there is a condition. The Sunstone that illuminated the clan's ship was aged and close to burning out. They ask Tal to bring them back another Sunstone, and instruct Milla to accompany him in this journey. Milla protests, still unconvinced that Tal is to be trusted, not to mention her longing to engage in a battle with him. The Crone informs her that this could help Milla in her dream of becoming a Shield Maiden. She eventually agrees, and she and Tal set out towards the Castle.
These are the "Masters of Light and Shadow", possessing magical control over Sunstones and shadows. Sunstones have their own light and heat. They are grown from crystals, or sun seeds, originating in Aenir. They are "charged" by exposing them to sunlight for extended periods of time. The "charging" is done above the Veil in large nets. The larger a stone is and the longer it is exposed to the sun, the more powerful it will be. They are a part of everyday life for the people of the Dark World, used in everything from giving light to cooking food. Chosen children are given weaker Sunstones, compared to the Primary Sunstone which older kids and adults use. However, a Chosen who loses all of their Sunstones is demoted to Underfolk status.
The Chosen live in a Castle with seven towers that extend above the Veil. A Chosen is judged by their color level, which shows their ranking in society. In order of superiority, they are: Violet, Indigo, Blue, Green, Yellow, Orange, Red (the colours of the rainbow), and the lowest level is Underfolk. The entire community of Chosen is governed by an Emperor or Empress.
Admittedly, this does gibe with the
tv tag from the querent as the series has never been turned into a film or TV series although Nix has started the process of licensing before.
Q. Will there be a movie of Sabriel and the other books in the trilogy?
Maybe. This isn’t something a writer can control. There’s always been interest in my books, but to date this interest has not crystallised into the required commitment to make the films, which would not be cheap or easy to produce. Sabriel has come very close on several occasions in the last five years, with various last-minute issues derailing it. This is typical of the film business. Shade’s Children and The Keys to the Kingdom series have also been through the hoops for film and television adaptations. So you never know. I have written a screenplay adaptation of Sabriel and I stay open to the various approaches. My film/tv rights are represented by Matthew Snyder at CAA in Los Angeles.