I think,,,and I'm using this term lightly,,,think that I can now sex B/S chicks at about 2 1/2-3 weeks of age,,the very 2 first eggs I hatched was both Opal B/S,,I have decided to keep them (had visitors and buyers here all weekend trying to pry them away from me) but they are for the most part still all yellow feathered,,nothing has really changed on them.Yesterday tho I sold a OBS chick that had significant coloring coming in the flight feathers(kinda lite rust colored) and some darker feathers emerging on it's back behind the neck.This particular OBS chick was 2 weeks younger than my first 2 that both hatched on May 20th,,,I also sold 2 Midnight B/S chicks,,and both of them was getting rusty colored wing flights,as well as color in the back..
My problem was at one time,,if I had several like colored chicks due to hatch at the same time,and wasn't there to witness who came out of which marked egg,,it could take months to see a diffrence,and on some B/S adult hens I have,,There is NO color diffrences,,so I have to mark them at hatch.I now put cool whip containers in the hatching tray and put like colored eggs in seperate containers,in case one of them pips-zips-emerges very fast when I may be gone,and I've had this exact scenerio happen before in less than 2 hours.But I think on B/S peachicks,,about week 3 you will notice feather color changes between sexes,,,I just hope my assumptions that the male b/s peachicks do get coloring-pigment first,,