Do the seed producers have a plot of these petunias for each color, which is mixed together and sold as a package?

Generally, yes. And they must be substantially isolated from each other to avoid cross-pollination if you wish to save the seed.

Would saving seed from a white plant, lavender plant, etc., (without knowing the pollen parent), be good enough to preserve the different colors? In general, yes but only if isolated if you want to be 90% sure. You'd still get some insect crossing but it would be a small %. You can control it to a larger degree by bagging some blooms and hand pollinating them and saving seed only from them to replant.

As to reverting, that will all depend on how stabilized the color variety is. With self-seeding, darker colors tend to fade and lighter colors tend to go toward white over time. So a regular infusion of new seed of that color will help it stabilize.

Hope this helps.

Dave

Thanks Dave. I was afraid that would be the answer.

I have watched four o'clocks over about a 15 year period. They are almost invasive here, but are pest and disease free. What started out as plants of at least four colors, is now reduced to two colors (with yellow barely hanging on, and the majority of plants a sort of pink color).

I guess this will be true of almost all plants.

Thanks
John