Yellow cotyledons on my seed starts

So I started 72 cell tray of kale collards Swiss chard and PAC Choy the first week in April . I used organic potting/starter mix. I also ran some dead leaves through a food processor to make a fine leaf mulch which I thought would help keep them moist ...and it did. I keep them in a warm spot outside during the day and bring them inside at night because nighttime temps are still dipping into the high 30s here in NJ. Day time highs have been in the 60s with a few days reaching the 70s. In the past few days it seems that all of the cotyledons have turned yellow and some even appear "dead" although the first new leaves appear healthy and vibrant. Is this normal? Is it too much direct sun during the day? It's definitely not too little water. Could this be disease from the leaf mulch I made? Here are some before and after pics. Is it ok to transplant them in this condition?

More pics

As long as your true leaves are healthy I wouldn't worry too much about it. Cotyledons often wither and die once they are no longer needed. If you start seeing clorosis in your true leaves then you have a nutrient diffidence. Probably available nitrogen. Too me those guys look 3 or 4 sunny days away from being transplant candidates.