For the original poster, trying to breed the reddest possible chicken without the crazy gene splicing stuff, I think your best bet is to get into New Hampshires, like others have said.
Maybe Moonshiner can confirm if there are other breeds that are redder, but I think New Hampshire breeders have already been trying to get the reddest possible chicken within the current genes available. They are gorgeous, but to me are still orange or rusty red rather than true red. (not my chicken in the pic)
Maybe Moonshiner can confirm if there are other breeds that are redder, but I think New Hampshire breeders have already been trying to get the reddest possible chicken within the current genes available. They are gorgeous, but to me are still orange or rusty red rather than true red. (not my chicken in the pic)