Yellow leaves could be a number of issues. Most common is too much or too little water. Try for consistent  moisture by feeling the soil down one inch and watering when dry. Too much hot weather after a transplant will send the plant into shock but it should recover. Lack of nutrients shows up as yellow leaves so be sure to fertilize with a complete plant food like Osmocote that includes all the micronutrients such as copper and magnesium. The leaves look a pale green above the yellow so your plant may need fertilizer. There are also  several tomato diseases that start off with yellow leaves. Fusarium root rot, leaf blight and other bacterial and viral infections. Rarely can the plant be saved from these diseases. Black marks on the stem and rot that spreads through the plant are signs of disease. I am going to be optimistic and hope your plant just need a dose of plant food and more careful watering. In a few weeks it should look great. Keep growing, Marianne Binetti