First time grower. Looking for suggestions.

They look quite nice and growing fast, leaves are developed nicely for only 6 days old. Hence, don't worry too much. The yellowing doesn't strike me as a severe problem. Few things I noticed: night temperatures at 63f is a bit on the low side. Try to get a few degrees higher, around 67 of 68f. Second: I'm not familiar with your lighting, but are you sure 12 inch is far enough? LED can be very powerful and seedlings are very sensitive to too bright light. Don't go from stretching to burning your seedlings. Try to find out what the recommended height is for your LED light during seedling stage (lookup information on the brand, company website, forums, here on GD, etc.). As for watering: the ground looks pretty wet. Make sure you don't water until the top layer is dry. Looking at the size of your pots and given the temperatures, RH and the stage the plants are in, I'd says it's going to be at least 2 days before you can water again, but probably 3. For example: Day 1: water, 2: nothing, 3: nothing, 4: nothing, 5: water. Happy growing!