Cosmos seedlings - yellowing

Just over a week ago I planted a fair number of Cosmos seedlings of three varieties - they germinated within two days and are growing very well and look healthy. They all still have ONLY their first "leaves" (Cotyledon I believe such leaves are called) but I've noticed that a few of them have turned yellow. None of them have their main leaves yet.

The vast majority are fine, nice and green, but I'm puzzled why some would turn yellow, maybe five in total out of maybe 80 or 90 seedlings. They're not drooping and besides the yellow colour they look okay.

Of course, it may be that they were yellow when they germinated and I simply didn't notice given that I have so many of them.

They are all treated exactly the same - good light on a south facing window sill (the sill has a radiator underneath it which comes on first thing in the morning and in the evening), watered but not overly so and not allowed to dry out.

Any thoughts please? Is the yellowing of some a sign of a problem?

 

Poor nutrient level in seed compost? A really small dose of epsom salts may green them up .

 

Could be, but they are all in the same compost and the vast majority are nice and green.