Yellowing leaves can be the result of insufficient sunlight, underwatering the plant, a lack of nitrogen or magnesium in the soil, or a disease. Tomato leaves may turn yellow for a variety of reasons, and a few yellow leaves are nothing to be overly concerned about.
As the plant ages, some leaves turn yellow and die. The leaves at the base of the plant that are blocked from the sun usually yellow first. These leaves can be removed without harming the plant. Ensure that the plant is getting enough sunlight, water and nitrogen. Nitrogen can be added to the soil with tomato fertilizer or a foliar spray. If there are also spots on the leaves, this is likely a symptom of a more serious disease, and the plant may be lost.