Well, my seedlings always have purple on the underside. Alot of it depends on the type of light I view them under. When I look at my seedlings from above under any lind of brightish light, they look good. But if I look from underneath, not so much. I can start seedlings inside with 70 degree temps, or in the garage with 50 degree temps, and the underside of the early leaves are always purple.

I agree with 'edweather'. Mine have and are growing under ideal conditions and those have purple underneath.

Here are two of the dying seedlings plus two relatively unharmed ones, tho still undersized for 4 weeks.

I can't seem to focus in more to show the purple areas.

50F is too low to support healthy tomato growth.
After a seedling is stunted as yours are, they won't fully recover.

Yes, they may sort of recover, but not completely.
You will be disappointed with the scanty yield.

Oh, I've already ordered replacement plants. Those are toast. Just wondering if it was the cold or something else.

Are you fertilizing at all! they are very sensitive to fertilizer. Possible overwatering? In my case seedlings never just die, it's always something I'm doing wrong.

Nope, no fertilizer. I don't start that until they have their true leaves, which these never got. All I can think of is the temperature.

I do my tomatoes pretty much the same every year, and this has never happened.

I'm wondering if anyone has chilled their tomato seedlings, and if they look like this.

As was said, "50F is too low to support healthy tomato growth"

But have you seen tomatoes that have been chilled? Have you seen what they look like?

In the previous thread, some people were suggesting that temps down to 50 were acceptable.

Here is a link that might be useful: 50 degrees F for tomato seedlings

I don't think it's the temps. Cool temps usually make sturdy plants. What's that mix you are using? Looks like pure peat. maybe too low pH? I'm just grasping at straws.

No, it's mix, not pure peat. And I've used it for everything this year, nothing but the tomatoes have problems.

Looking thru the tomato forum, I notice that some people are claiming purple on stems is a sign of chilling. It's the only clue I can find.

What really boggles me is that the pepper seedlings are just fine - in the same mix in the same flat in the same location.