Short of nitrogen. At this stage, use a kind that you dissolve in water.

I don't think its over or under watering. When my plants look like that I hit them with the kitchen sink.

A little none organic fertilizer. I use Peters/Scott's 20-20-20 with Micronutrients at one forth strength.

Something with some Iron in it. I use MaxiCrop Plus Iron at one Tablespoon per gallon.

Something with Magnesium it. I use CalMag Plus at three teaspoons per gallon. Though in a pinch I have used Epsom Salt at a ratio of roughly a tea spoon per gallon.

I hove no doubt everyone is going to say I'm crazy but it works for me every time.

Thank you jean001a and thisisme for the reply. So I am not drowning them, I am starving them. I will give them some fertilizer as you all suggested and let you know how they are coming along.

Purple leaves can be caused by a phosphorus deficiency, or lack of P uptake due to high iron in soil. A small amount of lime can help make P available to your plants.

I don't know where the plants are growing -- house, basement, unheated garage, garden shed -- or what the temperature is.

Purple leaves can also be the result of cool temperatures. There may be plenty of phosphorus in the soil, but the cold prevents the plant from accessing it. If the temperature is the problem, adding phosphorus won't help.

The plants have gotten worse since I posted the pictures (more yellowing with the edges of the leaves drying up). I fertilized them 5 days ago with Peters 20-20-20 diluted to one third strength but have not seen any positive changes. I think my mistake was using standard potting soil to pot them up and then adding too much water. Even though I have poked many holes in the pots, the potting soil appears to be retaining the water and they are not getting a chance to dry out. To try to salvage these plants I am going to put them in the ground this weekend and cover them with Wall-o-Waters. Hopefully I can report back in a couple of weeks that the plants recovered.
As I am trying to learn from my mistakes, I have started the seeds for my main planting which will be in mid to late April. This time I will only be using Pro-Mix soilless and I will be very stingy with the water. I guess I should have paid more attention to the OSU and Rutgers seed starting guides: http://pods.dasnr.okstate.edu/docushare/dsweb/Get/Document-1377/HLA-6020.pdf
njaes.rutgers.edu/pubs/download-free.asp?strPubID=FS787

This is why I do more than just give them Peters 20-20-20. The roots are white and healthy. Try using something with more iron and something with Magnesium in it.

Can MaxiCrop Plus Iron and CalMag Plus be applied without adding too much water to the soil? You can see in the picture that most of the roots are in the bottom half of the soil. I'm thinking I need to get the nutrients down there without drenching the soil again.

"Can MaxiCrop Plus Iron and CalMag Plus be applied without adding too much water to the soil?"

Sure they can. Just mix a gallon combined with the proper amount of each of them and use as needed.

You may want to keep the later planting seperate to be on the safe side. When you mentioned "mushrooms" Timber Rot immediately came to mind. You really don't want that fungal disease. In the future you would be best to use only sterilized mix for everything.

Those roots look fuzzy, white and healthy to me. Sometimes having lights on for too long, especially in a colder environment, will cause stress and yellowing and purpling.

Update - those same plants are now 4 feet tall and loaded with green tomatoes. I put them in the ground under a polly tunnel in mid-March and they perked right up immediately. Thank you for all of the advice.

I am glad you plants are doing OK. I just saw this post and wondered if you had holes in those containers so they could drain.