So....I had left a few flowers on and gathered the seed. I stored the seed in the refrigerator and started the seeds in a few peat pots just enough to get one hanging basket worth of plants "in remembrance" of that summer.
I was told correctly that the resulting flowers would probably not be the color they were but purple. Each year, I leave a few flowers on to collect the seed, store it, and start a fresh plant in the Spring.
The question: Each successive year it seems the seed is less able to germinate. This year I tried almost 30 seeds and got 5 plants and 3 died at the 2-4 true leaf stage. I don't have this problem with other plants or with other petunia seeds. Is it possible for a plant to basically become more sterile such that any seeds produced just don't work anymore?