The brown 'more data needed' days are days where more data is needed for the algorithm to be able to determine your fertility status for that specific day. You will only see Brown days if you are in Plan a pregnancy mode. The important thing to remember is to keep measuring!
It doesn't mean that you need to enter more data today, or for previous days, just that we need more a few more days to collect more data to analyze.
Brown days are most often shown for new users, as the algorithm hasn't gotten to know your unique cycle yet. Brown days may also be shown for users with fluctuations in their temperatures and/or irregular cycles. Its sole purpose is to make you keep measuring so that the algorithm can get more data to confirm whether ovulation has occurred or not.
What you can do is to check your graph and see if you have any fluctuating temperatures that may affect the data negatively. If you are on your very first cycle, there's not much to do right now than to keep measuring.
Please read this article to learn more about how Natural Cycles detects your ovulation.