Red & Green days
Red, fertile, days
You should use protection or abstain from vaginal intercourse on a red day, as you are fertile during these days. Until your ovulation has been confirmed you will get red days, this is necessary for Natural Cycles to be an effective contraceptive. Keep in mind that it will take a few cycles for the algorithm to get to know your unique cycle and be able to confirm ovulation. This is why you'll experience more red days in the beginning - but don't worry, soon you'll get your first green days and after that they'll keep increasing.
Green, non fertile, days
You do not need to use protection. You cannot get pregnant as you are not fertile. Natural Cycles is backed by a unique algorithm that takes your temperature and many other factors like sperm survival, temperature fluctuations, and cycle irregularities into account in order to confirm that you're not fertile and give out green days.
! Please keep in mind that the only method of contraception that protects against STDs/STIs is the male and female condom. Should you have reason for concern, be sure to use them regardless of fertility status.