Hey stoner chick, I wouldn't worry to teribally much about the yellowing. You plant looks pretty healty to me. My little seedling had a MUCH worse start, with a bad burn on it's first true leaf! But it kept on chugging! check it!
This is at a couple days old (the damn Miracle Grow soil did it)
and here she is 2 weeks later
And yours didn't look nearly as bad as mine! Look at the botom leaf of the plant in the last pic, there's the burnt leaf. So I think you'll be fine just keep an eye on it... I am growing in soil tho, I don't know anything about growing AG. hope this puts you at ease a little... lol