The damage looks like edema to me. For some reason I had this badly last year when recently potted tomato seedlings were moved under the LED light - that is not a plant light, just an LED shop light. Apparently edema is a common problem under narrow spectrum LED.
If your seedlings were raised under them from day one without a problem, it could be that you triggered the edema water-uptake problem while trying to tweak the temperature with heaters and fans.
If I were you I would try to get them into natural light as soon as possible. I would not worry about temperatures in the 60's for tomato seedlings, and a bit of leaf purpling is not harmful. Exposure to cooler temperatures will activate cold tolerance genes so that the plants are better able to cope with outdoor conditions. The edema OTOH is not good, it can cause damage that can't be repaired and it may take a while for plants to recover. The worst affected of my seedlings were stunted for some time after and although they did eventually return to normal, they flowered much later than their normal time. So you don't want to let this continue or get any worse.
