You'll have zinnias from sowing their seed, that much you can count on. ;)

Not likely you will get exactly the same height or color range from all seedlings, they will have cross pollinated -
if your plants were grown from hybrid seed, they may not have come true even if you were growing only one color.

Here is a link that might be useful: Photo ripe pod and zinnia seeds

There are many discussions about zinnias on the annuals and seed saving forums, if you'd like to read them.

I usually get annoyed when someone says go look somewhere else but this time I think the suggestion to go to Annuals - GardenWeb is a good suggestion. There seem to be a lot of zinnia enthusiast cluster there.

I don't usually tell people to "go look somewhere else."

There is so much info about zinnias, a most unusual plant, on the other forums, I would have felt compelled to paste several entire threads here to answer OP's questions.