Hi,
You will get flowers that are different. I had one wave type reseed and the flowers were dinky. Waves are hybrids. Hybrids sometimes have complex breeding to achieve a desired trait/traits. So lots of genes go into the making of wave petunia and some express themselves in a bad way in the offspring like my dinky flowers. Now that is not to say you won't get nice babies too, but it is a gamble. If you have room or like to experiment, you can grow out seeds to see what you get. If you do get a desirable baby, you can get rid of the other plants, keep growing the one you want and save seeds from it. If you keep doing this for a few years, you can develop a variety that will come true from seed. Then you have your own variety that you can grow every year. Doing this is called de-hybridization.
Remy

There are some waves that will come like their parent ie the Laura bush petunia. But still others will be different than their parent the petunias are open pollunated so it is hard to say for sure what they will turn out.

Just because you plant all purple does not mean that a bee or other insect or whatever did not visit your neibhors blue or pink petunia.

Try your hand at saving and then growing your own petunias if you do not the like the flower that it produces you can always pull and chuck the plant(s) that you do not like and not feel bad since it is a "bonus" plant since you collected the seed for it.

You might discover that you love the mix of colors that come up.

Paula