Hi Mary,
It all depends on the cultivar you have and whether any other varieties are grown close by. If you have another color yarrow blooming at the same time, they will cross. The seedlings will have variations in growth and color. If no other varieties are close by and it is not a hybrid it will come true from seed. Like if it is the straight species Achillea filipendulina which blooms yellow, your babies will be exactly the same. A named hybrid(I don't know how often this is the case with yarrow) will have seeding variation just as if you had another cultivar growing close by.
Remy

Remy, Thanks so much for the info. Yes I have other ones close by. I appreciate your answer!