This is easy, and you can keep all the hens if you have the room, for their eggs. But be very selective in which eggs get incubated!
When you are cracking eggs for eating, check how many are fertilized. Your rooster may be doing a super job, or not. Hopefully most will be fertile, and you...
If you plan to hatch eggs from this flock, only hatch from the darkest eggs; that's how you can develop the darker colored eggs you are looking for. If possible, ID the cockerels from those darkest eggs as possible future sires.
Mary