It depends on the character and how well-written their story is, but I prefer green flags, except for the occasional red flag, as yanderes are my weakness.
Since I started playing otome around 10 ish years ago I've managed to consistently like the green flag dudes - although there are well written characters that are absolute anomalies to morality, I.e. yang. He was written SO well that I think he's been the exception so far
I love character development so I tend to prefer red flags bc they have lots of room for change. I loooove a red flag becoming, well not necessarily green but maybe a yellow flag by the end of his route. A green flag usually doesn't have as much room for change bc they're already green and that's a bit too bland for my liking.