Hey man, Im also having this happen to one of my seedlings.. 2 are very nice dark green, but the oldest one (by one week) or so, has yellowed leaves as well..
All of them, the baby leaves, and all others..
Its only starting on its 2 set of true leaves.. So Sorry I cant help you much myself..
I am only using a B1 vit., but went to just water for this one, to see if light flush might bring some color back..
My soil's pH looks to be about 6.5, using water thats 6.3.. My 2 other seedlings just came up 2 days ago, so they look to be in great health so far.. No signs of yellowing..
I found this page that has alot of tidbits, of various prolems, and solutions..
Nutrients/Bugs/Various Problems Guide - Grasscity.com Forums
I dont think its a nitrogen problem since they are so young, and also think they are too young to give any nutes..
I just added a pinch of used coffee grounds, just incase it needs some nitrogen..
Im using some soil that just said organic on it.. I didnt add anything but perlite, and vermiculite..
Using 75w cfl's, and b1 solution from liquinox..
I just moved overseas, and no real garden shops, or grow shops insight...
Been here for almost a year now.. Could only find the basics.. Hints the used coffee grounds..
Getting the proper nutes, and things has proved to be pretty hard..
One thing I read on the link above, is that overwatering could be a cause, but the plant isnt wilting at all..
I have a fan on them all the time, so its not too hot, and can keep lights as close as possible..
So the soil does dry out pretty fast. I do water them when the soil is dry on top, and a bit below the surface.. However now Im only going to water 2 times aday.. WHen I wake up, and just before bed, so I can see how it helps or not helps on this particular seedling..
Maybe you can get some use out of the link
I guess Im just going to try a few things, and see how trial, and error works out..