Why does Lazet's hair color suddenly change?
Because


   She is not a human, but a Noel Mermaid, who has an ability to change their face's appearance. Her original face is the one with pink hair and eyes.


So, when Teito met Lazet,


   She mimicked Teito's face: brunette hair and green eyes.






References:


07-GHOST Wikia: Razette