They germinate (self-seed) readily from seed. End up smothered like weeds, especially the white ones. If you have several varieties in proximity they will cross-pollinate and you might wind up with some lovely colours. Just be patient and a little untidy and don't hoe or weed nearby. Transplant when the second or true leaves appear.

Just be a bit careful if you're collecting the seeds and handling them. They often cause skin irritation.

as said the seed will be a good source of some variation. if you want to propagate this plant so you only have this type then you will need to lift and split the plant. it has quite brittle roots so lift and split and replant immediately. remove the old leaves and water well. the plant will clump up quite quickly.

welcome to GoY too :o)