Could be a variety of issues. Potassium deficiencies can manifest in a number of different ways, but I haven't seen something exactly like this. Usually the undersides of leaves (or veins) will turn purple--or the periphery of leaf tips will. Adding a bit of potassium (e.g. as a thin dusting of wood ash
) won't hurt anything. Check your soil pH, though, as being too acidic or too basic can cause troubles with nutrient uptake.
If your soil pH is good (6-7), and the plants don't respond to wood
ash, you may have other issues. Has it been quite hot/dry? This can slow nutrient uptake, even with adequate watering. Adding some temporary shade (even a bedsheet thrown over the tomato cage) during the hottest part of the day can help plants perk back up.
Nitrogen and calcium deficiencies can also, in some instances, cause purpling. Waterlogging and cold can also do this. Try amending the soil in each way--give it a week. If none of those works, then consider pulling them out and burning them, in the event that they do have a unitentified disease. There is next to nothing published recently on purple leaf disorder--this is unusual for emerging plant diseases (the initial University of Florida publication was in 2008). It's possible it's what you have--a probable virus transmitted by whiteflies (and possibly aphids), in which case you should
destroy the affected plants to prevent the spread of the condition to your remaining tomatoes. Also consider applying a pyrethnum or soap
spray or other method to remove any whiteflies that may be present, as they will be potentially infectious.