I'm trying to find a book that a friend read when she was at school but can't remember the name. It has an underground school and they can travel to different worlds. There were different districts that were colour coded. She thinks it had an orange cover. she would have read this in the 2000's. Can you help?
Could it be the seventh tower series?
The world is shrouded by a veil casting it all into darkness, so it come across as underground. The book is set in a castle inhabited by people who do magic with light - the colour of light used denotes the power of the magic. Districts get assigned colours according to the colour of the magic of those living there. The magic is done with glowing stones taken from another world - which people in the castle can cross to.