Experts and BHG readers answer.
light leaves on petunia plant
It's likely your petunia is suffering a bit from cooler than average temperatures. Once the temps warm up it should green up. However, if these plants are jammed in one pot together it might need some fertilizer to get it going. I'm guessing the problem is the weather, but if you want to sprinkle in a little dry, slow-release granular fertilizer such as Osmocote go ahead.