2 of 5 added 72 characters in body

Trying to remember the name of a series I saw around 15 years ago

I sadly cannot remember much about it except that the magic system was based on a spectrum of color, there were two main characters (one male one female), and the second book started with them offering a blood sacrifice to bind storm spirits to them on a hill.

Anybody have any clues at all?

Edit: TV Tropes saved me: It's the Seventh Tower series by Garth Nix