Seedlings turning purple/yellow

Hi! I’m a novice tomato grower but my kids and I started off plants indoors this year. We planted seeds early in March and until this week they seemed to be doing great. Now the lower leaves on these plants (all 3 varieties) are starting to turn purple on the bottom and sort splotchy yellow on top. One leaf has almost shiny dots in the yellow area. Some up them are curling lengthwise bottom over top. I’m sure there’s at least one thing I’m doing wrong, but would appreciate any help so I right the ship as soon as possible. Here’s all the details I can think would be useful:

• Happy frog soil after seed starter
• distilled water so far (we have a salt-based water softener)
• temps in the room between 70 and 80
• humidity between 30-35
• solo cups have 4-5 holes for drainage
• 150w viparspectra light, canopy getting between 700 and 500 PPFD 16hr/day
• watering roughly every other or every third day when the first inch+ of soil feels dry and the cup feels light and the dirt on the bottom sounds dry
• have not used any fertilizer yet

Thanks in advance!!