Yea, it's mostly the color, but the size is also useful for some of those breeds with naturally dark skin. Your cockerels will turn red something like 3-6 weeks, but your pullets will turn red when they are about ready to start laying, which is something like 18 weeks.
As for size, it seems to...
1. Sapphire Gem Pullet
2. Same chicken as number 1? If not, going to need a head shot to determine sex
3. Barred Rock Pullet
4. Dunno breed, maybe some kind of mix? pullet