Leaves turn yellow for many reasons such as too much water and poor drainage, cold nights or lack of fertilizer. Using a slow release plant food like Osmocote around the base of your tomato plants will insure that they are getting enough nitrogen to keep their leaves green. There are also disease and insects that can cause yellow leaves. If you send a photo of your plant I would be able to make a more accurate diagnosis of the problem. Keep growing, Marianne Binetti