My two year old flock, LF brahmas, actually hang out by color - lights with lights, darks with darks, and buffs with buffs.
With the week old chicks in the brooder it's mixed. I know the faverolles seem to hang together, two of the NNs are the baby flock leaders and are always looking for trouble to get into together. The other two NNs seem to do their own thing, sometimes mixing with the easter eggers. The speckled sussex hang out with the easter eggers, as does the one silkie. The other silkie, I'm convinced it's a cockerel, is in love with one of the speckled sussex. He follows her everywhere. Hate to break it to the little guy but she is seriously going to outgrow him.
My Columbian Wyandottes all hang together, but they are new to my flock so I thought that was why. My original flock is all banties and they have always stayed together, even though they are all different varieties. I have one Barred Rock LF that hangs with the goats
I have 28 chicks of various colors so it will be interesting to see where they end up. Right now they are just starting to free range so they are everywhere with everyone. I am a little overwhelmed with them all. I wasn't going to keep them all, but I ended up having Marek's in my flock and I vaccinated late (not knowing for sure that I had it) and now I don't want to infect anyone else so the chicks are ALL mine.
It will be interesting to watch how they all pair up.
All of my chickens do the same thing with self segregation by breed. Even when I put my pullets in with my hens the pullets still group by breed and the hens seem more tolerant towards their own breed. I have also noticed that my ducks and the turkeys do the same thing. They all get along fine but the prefer their own breed.
My two sussex are always close together. I wonder if they stick close by color or breed, to help create a sort of camaflauge? A light bird next to a dark bird would be pretty easy to spot, but a group of dark or light birds flocked together might make it harder for a predator to single one out.