I just noticed that some of my tomato seedlings have their first set of true leaves starting to turn yellow, with a few dark green spots clustered around the veins. The underside of the leaves on all the plants are a dark purple color, which I can't seem to get a straight answer on whether that's a problem or not.
I've got them under a grow light which I keep turned on for about 14 hours a day. I water them every other day for the most part, and I just started adding liquid fertilizer diluted to 3-1-2. I've only done this once so far but was planning on doing it once a week. Another thing to note is that the room I'm keeping them in averages around 67 degrees, and I've been a little worried that this is too cold.
Do any of these things seem like an obvious red flag? I recently lost all my pepper seedlings to damping off disease so I don't want my tomatoes to die too :( Any help would be greatly appreciated.