Last season I had a problem with brassicas viewtopic.php?t=5301. I figured out that they're extremely sensitive to being wet, and that both of the soils I have used stay wet way too long. It also seems that the more you water, the length of time the soil stays wet and doesnt dry out increases a lot. This year is much better.

Anyway, this year the problem is with larger tomato seedlings higher than about 8 inches. The lower leaves are turning yellow, purple, and curling. Upper leaves are healthy or relatively healthy. I don't think fertilizing or watering is the problem. I think it's light. I have t8 fixtures with a mix of LED and fluorescent bulbs, and the LED bulbs are the unknown as usual. Things were much simpler and predictable with just fluorescents. I'm playing with distance, number of bulbs, and length of time they're on. Any thoughts are welcome. I've attached photos of unhealthy and healthy leaves, and a healthy small tomato seedling and pepper seedling for reference.

Looks like edema @Salaam .
Lights do contribute to the problem, so if you can move the affected plants onto a windowledge where they get natural light, it may help.
Managing water is also crucial. If the soil gets dry and is then flooded, these plants can't adapt to the change of conditions.
It's a real problem with potting mixes in indoor temperature and RH, where it may be difficult to keep them evenly moist without actually overwatering and making them waterlogged.