Tomato Leaves turning purple is a cause for concern. The most common reason this occurs is a phosphorus deficiency. Tomatoes are heavy phosphorus feeders.
The most common and logical reason for a phosphorus deficiency is simply that there is not adequate phosphorus in the soil, but it is not the only reason. At times there may well be enough phosphorous in the soil but the plants are unable to absorb it, an 'uptake' issue.
Phosphorus Deficiency Causes Purple Leaves
Phosphorus deficiency is sometimes confused with nitrogen deficiency, it is uncommon, but does occur in areas with excessive rainfall or poor soil. Soil such as heavy clay or chalk soils.
Cold weather can cause a deficiency which is generally temporal. Symptoms include stunted growth, foliage that turns blue/green or Purple. The undersides of tomato leaves, as well as the veins and stems, sometimes turn purplish.
Older growth leaves are affected initially, followed by new growth foliage. See Also: Plant Nutrient Deficiencies.
Under cooler growing conditions tomato plants are sometimes temporarily hampered in their ability to absorb and use nutrients. Temperatures around or below 60 degrees will create this issue. Generally assuming your purple tomato foliage is related to cool weather, it will rectify itself as the plants mature and the weather warms. Do not over water as it tends to exasperate the situation.
If your plants have not been subjected to cooler temperature and still exhibit symptoms of phosphorous deficiency than your soil is probably lacking in that essential element. Phosphorous rich soil amendments of bone meal or Organic Miracle Grow are best as they are more easily and rapidly absorbed by the plants than mineral sources of phosphorous.
Other Reasons Tomato Plants will turn purple
Psyllids are small flying insects, basically sap sucking plant lice. They are easy to spot as they are usually present in large numbers. Shaking the plant at times will send a cloud of them into the air. They inject a toxin into the plants which causes the purple discoloration.
The earliest symptoms of a Psyllid infestation is a slight yellowing and / or purple discoloration of the upper leaves. Lower leaves tend to curl slightly upward. Under a severe infestation the entire top of the plant changes to a reddish purple and sometimes yellowish green.
Curly top virus is another reason. It is caused by Leaf Hoppers, symptoms include a purplish color of the veins and leaf undersides as well as chlorosis, yellowing and distortion of some leaves.