"have them directly under a flourescent light, approx 6-8 inches above"; This is WAY too far away. Florescent lights should be no more than an inch away from the plants. If you are using t12 tube floros the plants can touch the lights with no problem.
I'm also guessing that your seedlings are 'leggy' due to the light being so far away. The stem from the soil to the cotyledons should be an inch or less tall.

Jon

Also if you have them in egg cartons, I'd get them transplanted in pots. The roots will be as long as the green above the soil is tall. Those roots probably need some room to grow.

My little tomato seedlings are developing true leaves, but as in past years, the stems and under side of leaves are purple. These seedling are in Burpee seed starting mix in peat pots. They are 1-2" away from fluorescent lighting and also get warm sunshine and breeze during a day when I can put them outside. The few that I have in tiny expandable net pots do not have any purple. I read somewhere this purple might be due to a deficiency of some element like potassium or something. Any suggestions?

The purple is phosphorus related, not a deficiency but an uptake issue and is actually quite common for young seedlings. Their circulatory system is still developing.

While it is more pronounced in some varieties than in others it poses no threat to the tomato plant and quickly disappears as the plant develops and goes away completely once they are transplanted to the garden.

Some say that using warm water to water the plants will help but I've never found it to make much difference.

Dave

No Jon they are not leggy because they are outside in a cold frame since 10 days ago where they receive sun all day. I just placed a light in there today because it is raining and they will not get any sun.