Well for me, it was a sign of my demise. I toasted my seedlings in MG soil, and the first sign was yellow cotyledons. Yes, they are supposed to die off, but mine died almost instantly, so it depends where you are at your veg growth.
I don't want to freak you out, but my very recent experience was that MG soil is too toxic to seedlings, and yellow cotyledons were my first indication.