Abnormal color changes on your tomato plant should throw up a warning flag for you. If you notice, after you have transplanted your tomatoes, that leaves are turning yellow, you should consider what might have gone wrong during the transplant process. If it happens soon after transplanting, it usually is a problem with the transplant rather than a new problem caused in the garden.
Lack of Nitrogen
When grown in potting soil mixes and pots that have been fertilized carefully, tomato plants aren't likely to lack for nitrogen. However, when they are moved from that controlled location into a soil plot and you didn't test the soil beforehand, you may be planting them unwittingly in an area with low nitrogen, which can cause yellow leaves. Adding some organic material or nitrogen fertilizer should help correct the problem.
If a plant isn't getting enough water, especially if the heat is high, the leaves may become scorched and turn yellow. This is a common problem in transplanted plants because the roots aren't anchored properly and drawing water the way they do in a more established plant. Ensure that your tomatoes get enough water at all times for a few weeks after they are transplanted to avoid this problem. Don't overwater though, as this also can cause problems. Damage of the roots during transplant also causes this problem.
Another cause of yellow leaves is early blight. This fungus is spread through many different means, and it is important to keep an eye out for it. If it shows up on transplanted tomato plants it is likely that the blight had been a problem there previously. The best way to avoid this problem is to rotate where you plant your tomatoes frequently. Crop rotation and destroying plants as soon as you notice blight help prevent large outbreaks.
If you transplant tomatoes into an area where insects already are a problem, they will quickly attack the new food source that you have introduced into their area. Many different insects cause yellowing of the leaves as they attack plants. Psyllids and aphids like to eat tomato leaves and will cause the leaves to turn yellow. Look on the bottom of the leaves to see if insects are present and causing the problems. Other insects causing leaf yellowing in transplants include spider mites and whiteflies.