I've been starting White Widow autoflower seeds in a one inch layer of coco coir that sits on top of a cup filled with Fox Farms Coco Loco, another soil that is reputed to run kind of "hot". I figured since I'd be transplanting into Coco Loco it made sense to start my seedlings in it. This has worked very well for me, but I did have one seedling (out of 12) that at about one week completely 100% turned this yellowish/tan color over a period of several days. It's growth significantly stunted for about a week and then over a period of several more days the seedling returned to a normal green color and coming into week 5 it is now one of my strongest performing plants. I can't total say it was the soil, because this only happened with one seedling, but I do wonder if it wasn't some weird combination of the this particular seed's genetics needing a bit of time to adjust to a "hot" soil.