Is the room you're growing them in sort of chilly? If so, your plants might have a phosphorus deficiency. The nutrient is hard to absorb in chilly temperatures.

They are on a heating mat about 3" under lights. The lights go off at night but the mat stays on the whole time. I have two other peppers - one bell and one paprika and neither of them had a color change. However it is in the laundry room in the basement so it's a bit chilly down there - 60° F or so...

I went down stairs and the air temp is 71°F - a lot warmer then I thought. The soil temp is 77°F.... The only other thing I can think of is I was running the lights all day and night (due to a busted timer). About the time I replaced the timer is about the time the leaves changed. Maybe it got too cold for them at night? I also just plunked a ziplock bag over their starting pot to help keep them warmer.

You need to take them off the heat mat - heat mats are for germination only, not growing. Purpling of leaves is perfectly normal in young seedlings that are stressed for some reason and if you search 'purple leaves' you'll find numerous discussions about it.

It poses no threat to the plant and goes away once the stress is removed. For most of us part of that stress is too cool air temps this time of year added to the stress of just trying to grow. ;) But for your plants it is too much soil heat for the young seedling's roots to cope with.

And please don't cover seedlings with plastic. They will tolerate cooler temps much better than the lack of air the covering them with plastic causes.

In the greenhouse the normal maintained air temp is 50-65 degrees. Much warmer and the plants grow too fast, get leggy, and the roots can't support the top growth.

Dave

Thanks Dave - I think you're right. I found this, "Once the seedlings have 2 sets of true leaves, they can be up-potted to a 3-inch pot, and grown at 70°F during the day and 60°F during the night." The air temp is pretty much 70°-ish F right now and the flats not on a mat have a soil temp of 73°F. So I've uncovered the Lady Bell and moved it off the mat along with the other peppers. It should be a bit cooler for it this evening too then. I'll post back with the results of this change.