Wow!  You do have some darker ones in there.  I can't answer your question about the totally brown ones.  I didn't have any with my orders, but I'm not experienced enough to know if that's common or not.  Hopefully, some one else will hop in and help you with that question.  
The ones with the chipmunk stripes are female and the ones with a yellowish or whitish spot on their heads are male.  
I have noticed some of mine are more grayish, others are more brown and reddish and even others are more golden.  I'm going to try to keep track to see how the chick coloring relates to adult coloring - if at all.
Enjoy!  They sure are cuties.