There is no telling a difference because they are young. Brown cere is a female but in some cases they are white blue when they are laying eggs. If its blue cere its a boy. Just wait until they are more older like 1 week more or until the ceres of each are noticeable.

I just bought two parakeets yesterday and I wanted a male and a female. The petco employees had no clue because the parakeets are so young. I did a little research online and I think I got a boy and a girl but I am not sure.