Birchen cochin bantam help

So, in the never ending saga of trying to ID our black chicks, we are down to 2 black bantam chicks that we are still on the fence about. I *think* the one I have in my group is a black frizzle that is only semi frizzling/curling. It has black feathers coming in on it's neck. The other black bantam looks to be getting whitish feathers but it's hard to tell. Both chicks are 4 weeks old. So my question is, when do birchen's start getting their colors on their head/neck area? It's causing angst amoungst our group b/c folks are starting to become attached.