Well, my summer of seedling horrors thread documents my killing of four Apollo 13 seedlings without being able to figure out what I was doing wrong, but the ChemD x Sunshine Daydream guinea pig used to test my soil survived and is 47 days into flowering. As for my soils, I’ve tried…
- Happy Frog
- Light Warrior
- Coast of Maine Seed Starting Mix
- Espoma Seed Starting Mix
- Coast of Maine “Platinum Blend,” cut with 50% coco
- Homemade seedling mix (1/3 compost, 1/3 coco, 1/3 perlite)
… there may have been others. Happy Frog and Espoma are the two that have given me the best results. Tested pH on most of the above and nothing was too out-of-whack (6.5 to 7.0). Also the 
part is that I use the same soil (mainly Happy Frog) for clones without issue… so if soil pH or nutrients were the cause, I’d assume my clones would be in the same situation.
Specific to the cotyledons, the Bingo Pajama has hints of yellow, the C99’s are completely crisp. Ugh. Guess I’ll feed them… I don’t have an immediate need for these plants, so if that’s not the answer and they get worse, so be it.