George, 
Where on the plant are you seeing this issue? Is it on all of the leaves? Leaves at the bottom, or newer leaves? This looks as though it could either be a nutrient issue or some type of pathogen. If you could send a picture with the whole plant that could help! 

Sure.  Its kind of all over.  Existing leaves.

Thank you, from that picture I'm wondering how frequently you are fertilizing your tomato. Containerized tomatoes need fertilizer more frequently than their in-ground counterparts as the nutrients "leech" through the soil with watering. There could be a deficiency in either nitrogen or potassium leading to that discoloration. 

If the yellowing becomes more distinct on the lower leaves and you begin to see dead spots develop, remove those leaves as you may have early blight, a fungal issue, developing. 

I hope that helps!