Why 02 Zero Two's color is Red and Pink? Recently I was watching DARLING in the FRANXX. Unlike all the other girls and boys. Zero Two's uniform is red. She also has long pink hair. 
I think the reasons are as below:
1. Her personality is dangerous and alluring. She's an elite pilot with the pseudonym "Partner Killer" (パートナー殺し, Pātonā-Goroshi), given the mysterious happenings of her co-pilots dying after a short number of FRANXX outings. She appears to be playful towards Hiro and sometimes shows some sense of humor; even so, she follows her duties. It is implied in Episode 7: Shooting Star Moratorium that she knows more about the outside world than the other members of Squad 13.
2. She is like fire. Once she confirmed that she should do, she will do it completely. She choose Hiro as her darling. She would insist to the end even this would hurt Hiro. That's what darling means. That's what red means.
3. Pink hair. She is so beautiful with long pink hair. She desires love, she desires to be treat as a normal human. Actually she is a normal girl.