Height and leaf color?

The seed powers growth for 10-14 days or so.. you are fine... if anything you want a very light soil mix or light nutes in soilless irrigation etc... If the soil is hot, it's possible that's why you see some discoloration, but they will likely power right through it, as it seems minor. I doubt it is a deficiency. if you add any fertilizer, make sure it is a light dose... familiarize yourself with the soil you used... if you stick with same brand and anything added, it should be similar timing on needing nutes. stick to good irrigation practices.. wait for top 1" to dry before irrigating agian. learn weight of a dry pot as that's simplest way to feel 'when.' In these little seedling trays i'd wait for top layer to dry and maybe up to 1/4". get a little runoff to ensure entire volume of soil gets wet. rinse and repeat and feed every other? every third? or whatever works well for your fertilizer mix. keep pH around 6.5-6.8.