I'm glad you asked anyway. They look exactly like seedlings I had about a year ago in small coco pots, including the one with the bad leaf. While it was in my case fairly sure initially due to overwatering - plants that stage don't need nutrients, especially as long as the cotyledons are healthy - that also flushed the buffer out of coco. I'm no coco expert but pretty sure you're supposed to avoid using water only especially not for a longer period as it flushes it out the buffer.
No, just saying, so you can exclude that. It has to get extremely poor, dark almost before there's not enough light to produce chlorophyll (which is an Mg atom surrounded with N, hence why they are important for "green") and then they would clearly start out limegreen/yellow in the middle.
Glad to hear they are responding well to the feeding.
One thing though, on the pics the coco looks wet but the rockwool cubes don't. When you use rockwool cubes in soil or coco they tend to dry out faster than the rest of the medium, and watering the medium doesn't always make the cubes moist enough. Till the point the roots became large enough to suck water to it. It seems to be the case with all of them though, while if it would be a watering issue there would probably be more variation.