Newb question about my seedlings

[exnavycook]

Hello all! New to growing from seed. I got 43 Cherokee Purple tomato seedlings going about 3 weeks. About a week ago I noticed the base of the stem and the under side of the leaves have turned purple.

I'm not too worried about it but was wondering what is going on with them. I germinated the seeds in a moist paper towel and put them in miracle grow potting soil. They sprouted out of the dirt in 2 days. I have a small grow cabinet with 3 cool fluorescent lights and 2 warm lights. I keep the soil moist but not wet or soggy.

The temp naturally maintains around 75-88 degrees and until yesterday had them full lights 24/7. I have started putting them outside in the shade for most of the day. here are some pics I took. Any advice would be helpful. I'm sure as they grow I will be asking all kinds of other questions since I'm new to this. This forum is a wealth of knowledge, glad to be here.