You can pop a plant or two out of the pot to check the roots (if they dislodge easily enough), to make sure they look healthy and not decaying. We suspect this is just minor and temporary nutrient deficiency, perhaps from leaching out any nutrients that may have started-out in the potting mix, or if the plants haven't been fertilized in a while. Giving them a dose (based on package recommendations) of liquid fertilizer now, plus starting the acclimation process for outdoor conditions, should help them grow out of it once they go outside for the season. Foliage might not regain a deeper green color even if a nutrient deficiency or stress factor is resolved, but that's okay as long as the younger/new growth looks more normal.

Miri