This amaranthus,
Not this one

Now, amaranthus being a wildflower, it's not unusual to find the green-flower and other colored flower plants sprout from seeds you've harvested from the dark red ones. But, as with other flowers, if you just keep harvesting the seeds only from the plants you like, eventually, you'll pretty much eliminate the colors you don't like.

Amaranthus can easily grow to several feet tall, and if you pinch off the tops, they will bush out and produce a lot of flowers everywhere on the sprouting branches.   They can be strikingly beautiful.