Mordant: Book 1 in the Power of Nine Trilogy#1

Darric, a merciless warlord on the planet Vard, has finally defeated his enemies, although the lengthy war has all but destroyed his own planet. He must rebuild and train an army strong enough to invade Earth before his own planet becomes completely barren. It will take decades of hard work to prepare, but if he can get his hands on ancient relics that can magnify his deadly power, he will be ready to attack sooner than anyone could imagine. 

Mordant was born into slavery.  She does her best to keep her head down and blend in, but she hides a dangerous secret; she has the colours of magic in her mind. If her masters find out about them, she will be killed. Soon her magic will become too strong to hide.  She must control it, or die. 

With the help of her mother and a rebel spy, Mordant is given the chance to escape. Suddenly swept up into politics and conflicts she barely has time to understand, she must master new skills as mage, sword-master and spy. But the more she learns, the more dangerous the missions become: somehow she must reach Earth and deliver the urgent warning that Darric and his ferocious army are gaining strength and getting ever closer to their goal.