The leaves will yellow on tomatoes if they get too cold, too wet or if the plants dry out. Feel the soil with your bare finger pushed down into the soil up to your first knuckle and don't water until the top layer is dry. It doesn't take a frost, just a couple of cold nights to upset the heat loving tomato and cause it to pout and turn yellow. Don't give up, the tomatoes on the yellow plant might still ripen in time for a summer harvest. Keep growing, Marianne Binetti