What's wrong with my seedling?

stop trying to choose how much water you give... that is determined by the substrate and size of pot... and due to variables is potentially different each time... you water until the entirety is wet... with soil you won't want a lot of runoff as that is wasted amendments from your soil, but you want 'some' to be sure there are no dry pockets that over time allow for the possibility of odd buidlups of minerals over time. light green new growth isn't a big deal as long as it fills in quickly. More than 1 deficiency and toxicity can cause paling of new growth, but if it doesn't coincide with other symptoms, could be related to fast growth too. This doens't look interveinal, so it eliminates a lot of the possibilities, if it was actually caused by ratio/concentration of nutes provided. but, fix that watering habit before it causes a major problem in a future grow... it doesn't have to happen everytime to be important to avoid.

There's nothing wrong here that I can see... new growth is always much lighter/more yellow that the older growth... Have no worries!