Red, fertile, days
You should use protection (we recommend condoms) 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. 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, so you may experience more red days in the first few cycles.
Green, nonfertile, days
You do not need to use protection*. You cannot get pregnant as you are not fertile. You may get your very first green days as early as in your first cycle, and up to roughly 40% green days. With each cycle, you will experience more and more green days as the algorithm is getting to know you. After a couple of cycles, you may receive roughly 70% green days in one cycle.
* Keep in mind that the only method of contraception that protects against STI's is the male and female condom. Should you have a reason for concern, be sure to use them regardless of fertility status.