Yellow on Cotyledon normal?

i'd wager the soil might be a little hot for the seedling, but it seems to be hangin in there. reducing VPD with the dome during seedling stage helps reduce how much the little plant draws up from the soil. so you are doing what you can to reduce it already. (still have to harden it off, but it'll be fine) did something with fertilizer possibly drip on the leaf? another possiblity. it looks fine though. make sure it isn't progressing and if not the cause has abated.