Sexing 7 weeks old chicks

[R3M1X]

Sooo my chicks always throw me for a loop. They are either late bloomers or quick growers. In March I hatched three chicks and was certain I hatched two cockerels and one pullet, but turns out (after they turned 2.5 or 3 months the black cockerel was actually a black pullet) her comb and wattle grew faster than my other hens and pullets had grown. Now she grew into her wattle and comb so she looks like a proper girl.
Now on to my new chicks that hatch in June...
So I had one or two that were greatly larger than the others to start off with. They had larger combs than the rest even as a week old chick similar to my black pullet. So I am starting to think I should try to sex them with their feathers instead? When I first was trying to sex them I determined that they were all pullets, but I also figured there is a large margin of error on my part since I am still learning.

Below: the black, light silver, and gray with dark head, also the blurry speedster but obviously that is hard to determine lol. I am confident the black one is a pullet. I think the silver one is also a pullet but with a large comb. The two dual colored are Sapphire Gem mixes (same with the black) they are the ones that have always had large combs and they look like cockerels but I just need confirmation. Though the best confirmation would probably be when they are older lol

Below: a better picture of the silver one.

How can I tell by using their feathers?

 

[Frazzemrat1]

The black is a pullet, the rest of the grays... if they're all 7 weeks then they're all cockerels.
Its too soon to feather sex, as they won't grow in their male or female specific feathers until 12 weeks at the earliest, when they molt out of their juvenile feathers.

 

[cstephens1987]

When you say they're sapphire gem mixes what do you mean? Was their mother an SG with an unknown father?

 

[R3M1X]

When you say they're sapphire gem mixes what do you mean? Was their mother an SG with an unknown father?

I have a Lavender Orpington rooster and hens and Sapphire Gem hens and bred them all, as well as bought extra pullets to balance out the male:female ratio, which is why there is a brown pullet in the photo.