Remember they were starting to pale a couple days ago as the cotyledons shrivel up its just more evident now that they are ready for their 3" pots.
Those 1st leaves will continue to yellow and shrivel in the heavier feeders and nothing to really worry about as the roots are getting their 1st exercise in feeding the rest of the plant after a fairly heavy dose of nitrogen (2.5%) and calcium (2.5%) with a light dose of potassium (0.5%),phosphorus (0.5%) and magnesium (0.5%)... see how the 1st "true" leaves have a claw at the end? They are showing it was a bit too much nitrogen for them. Also, see how their leaves have a balloon appearance? They are showing they are not drinking the nutrient water as fast as the plain water so the roots are staying on the wetter side longer than they are used to....transitions can be rough
If your really worried about them you can up-pot now for growth to resume on Friday/Saturday instead of Friday for growth to resume on Monday/Tuesday.....use the same amount of plain water: 2Tb if the soil is moist, 4Tb if the soil is dry concentrating on the outer edge of the pot