What are you using for potting mix? Your own homemade version or a commercial package.
If home made, what's in it?
If commercial, which brand?

There are multiple causes of chlorosis (yellowing) of plant leaves starting with water logged soil and ranging through nutrient deficiencies such as Nitrogen, potassium, Magnesium, Zinc, Iron, Sulfur, Boron, copper, Manganese, or Molybednum.
I would start by taking a good look at the potting mix.

Thanks,

I use commercial mix....promix as a matter of fact.

This plant is inside my greenhouse and only the petunias seem to be unhappy. This one is the worst.

Pro-mix should be giving you the correct drainage - assuming your pots have adequate drainage holes...

Apparently petunia (which I don't grow) are part of an 'iron-inefficient' group of plants. You might get some ideas here:

Here is a link that might be useful: Chlorosis greenhouse bulletin

Pro-mix will give you the correct drainage only if you are watering properly. Sometimes you will find a few petunias like that in the midst of a perfectly healthy bench of petunia flats. Usually you will find the roots compromised if you pull it out of the soil and look. Often it is in a cell you find stays wet from too frequent watering and get automatically watered again, whether it needs it or not.

I find that if you pinch off a plant like this, and resume proper watering techniques and letting the soil dry in between, if the roots aren't too far gone, it'll recuperate. The old growth won't green up, but the new growth will be normal.