seedling help

need a bit of advice with some Melocactus discoformis seedlings,i planted two lots a couple of days apart,at day 3 after germination all the green seedlings of both lots have turned rusty brown although they appear to be still growing,are they dead? if so does anyone know what turns them brown,they were germinated in 50/50 cactus mix/sand in take away containers with daily watering,the lids were taken of at germination
thanks for your help

Perhaps too much sunlight. That will turn seedlings a reddish color also i leave them in a closed environment for up to six months usually in zip lock bags or take away containers i also have additional shad clothe covering them.

Hmm interesting. I planted some copiapoa's they other day and some of them have also turned a brown colour,whilst others in the same tray are vibrant green. They are still plump and growing, just an odd colour.

Yes it sounds like to much sunlight, it can slow there growth down though.