No problem.

For me the choice would only exist if 

A) They are obviously are of a mixed race

B) They are raised by both parents so they KNOW they are a mixed race

C) Their parent tells them they are mixed and force them to make the choice

D) Any mix of the above

I would think that for a mixed race that comes to the point where they have to make the choice, someone other than the parent/s will have figured it out by then and they will tell the leaders, and they will have to be informed of said choice. For those in the resistance I would say they would have made a public choice, and in the resistance they feel more free to be themselves.

As for the night and day thing, yeah, for the most part I can see it related to the actual time of day, but I also like the exceptions. Fairies like to operate at night, but I see them as Daywalkers, know what I mean?