Loks like a nutrient deficiency. I believe purple = lake of phosphorous.

Yep:

Purple leaves:
Leaves with purple veins and overall purplish tint. Early season, cool temperatures. No insects present Phosphorus deficiency

Put some tomato fertilizer on with a higher 2nd number. (Like 10-10-10, but rather 3-7-3 or something)

There seems to be a bit of everything involved here - mainly stress, accompanied by over-watering and possibly nutrient uptake problems. Can you tell us what were/are the daily highs and lows?

Thanks for the replies. These plants are in potting soil and I did not fertilize until 5 days ago (they were planted for ~2 weeks before the fertilizer was applied). I used 12-12-12 organic fertilizer and applied according to the instructions (1/3 cup on surface of soil, watered thoroughly). I thought that the potting soil had enough organic material to sustain the plants and fertilizing might not even be necessary, but after seeing the plants do poorly I decided to fertilize.

The weather during the first 2 weeks after transplant was relatively cool: highs 65-72F, lows 38-50F. On most nights when the temperature dropped close to 40 I moved the plants into the garage. There was one night when it got much colder (38F) than what was forecasted and they were left outside.

LIquid ferts are much sooner available to plants, especially growing in containers. I always keep some fish and seaweed liquids on hand.

That temperature mostly explains it. Even 2 consecutive cold days can lead to plant showing symptoms of nutrient deficiency because uptake is inhibited by low soil temperature. Its ok that you've fertilized, but keep in mind that the deficiency you are dealing with is not caused by the lack of nutrients in the soil.
What's the weather forecast for the upcoming days? Since you can move the containers, i'd put them inside whenever it gets to 50 or lower. Don't fertilize them for awhile, don't water them until the soils dries out a bit (presuming it has proper drainage), and don't prune anything yet. Once it gets warmer (57-58 and up), uptake should be fine and your plants will get better.

Thanks for the response and reassurance! Good news is that the 7-day forecast shows highs ranging from high 70s to 90F, and it's not supposed to drop below 56. I guess I was too eager to get them planted this year!

This picture looks exactly some of my tomato plants. I was concerned that they had a fungus so I pulled off all the lower leaves that looked like this. Oh well. I am wondering what organic stuff could I use to get phosphorous to my plants. Isn't the 10-10-10 stuff not organic?

The three numbers in a fertilizer description tell you the amount of Nitrogen, Phosphorus, and Potassium (otherwise known by the chemical symbols N-P-K). Note that the three elements are in alphabetical order: that's how to remember which is which.

The numbers refer to the percentage of each element in the fertilizer. So a 10-10-10 fertilizer has the same balance as a 3-3-3 fertilizer -- but the first has more of each and so would probably cost more. 70% of the first and 91% of the second are something other than N, P, and K.

As for why plants need these elements, here's what BHG has to say:
http://www.bhg.com/advice/gardening/basics/what-do-nitrogen-phosphorus-and-potassium-that-are-contained-within-fertilizers-do-for-plants/

momof5sons: Sorry, I forgot to explain the bit about organic fertilizer.

Whether or not the fertilizer is organic depends on what the manufacturer used to make it. Some 10-10-10 fertilizers are organic, others are not.

Here's a thread on the Organic Gardening forum which talks about organic sources of phosphorus.
http://forums2.gardenweb.com/forums/load/organic/msg0216391932711.html