Usually, the yellowing of plant leaves indicates excessive water so that is the best bet given your weather and watering pattern. But without seeing a picture of the plant it is impossible to tell for sure so check out the disease link below for comparison to your plants.

The best way to find out if that is the problem is to cut back substantially on the watering and note if the plant's color improves. Keep in mind that overwatering is far more lethal to plants than underwatering so when in doubt - wait. The roots need oxygen as much as they need water and excessively wet soil deprives them of that oxygen.

The best method to check is to stick you finger deep into the soil and check to see if it is dry deep down. Or allow the plant to begin to droop slightly before watering. They will tolerate that with no problem.

Plants in containers may also need an extra boost of food as much of it leaches out with the water or heavy rains. So try a 1/2 strength dose of whatever your favorite fert is too. Good luck and enjoy your tomatoes. Remember - they're tough AND tolerant of us! ;)

Dave

Here is a link that might be useful: Tomato Diseases