Three things spring to mind:

The supplier changed the ingredients in the grow medium

Purple leaves could indicate that they were a little chilly.

Perhaps your light bulb needs to be replaced.

Linda

Purpling of leaves in young seedlings is almost a given and it is a phosphorus deficiency (not potassium ) IF there is actually any deficiency which there almost always isn't. It is cool air/soil temp related and age of plant/root development related. Search "purple leaves' here for many, many discussions about it. Also lots of info about it over on the Growing from Seed forum here.

Trying to fix it almost always makes it worse instead. Instead focus on encouraging root development rather than stimulating top growth and don't worry about it. Of the couple thousand tomato transplants we grow every early spring 90% of will have some purpling of the leaves and it all goes away on its own as the plant matures from the 'toddler' stage.

Dave

yep, live and learn.
Purple leaves by themselves are not a big deal. But there are quite a few tricks to growing stocky healthy seedling that set fruits earlier and produce better.
Look under post 7 in this link, reprint from another person, Ami

Here is a link that might be useful: cold treatment

Thanks Dave, I knew what I meant, I just couldn't type it!! Phosphorus deficiency. Bulbs are two year old fluorescent growlights. But the temperature thing is a distinct possibility. I grow them in the basement because it is cooler, and with our extended polar vortex this winter, it may have been cooler than I thought. Timing would seem to be right also, as soon as they get their first set of true leaves I turn off the heat mat, and things get much cooler. Also, I know fluorescent output is pretty highly temperature related, could my cooler temperatures have impacted light output as well? I read through the attached thread, thanks lindalana, and once again, more reasons why I really need a greenhouse!!

Steve

Something I did this year for the first time, was add Azomite to my soil. The plants responded very positively. Azomite can be hard to find in some areas. I got mine at a hydroponics store. A little goes a long way and seems to work wonders for plant health. I applied it once in early spring, just before I planted the seedlings outdoors.