First grow

It is common for new growth to be a lighter green. If it becomes almost a yellowish lime thou it can be the signs of a nute def. Starting. But from everything I see that is not an issue. It's a great looking girl! Keep up the good work! Cheers and happy growing!

best guess is the 6.0 pH in soil. Bring that up to 6.5 and test that out. Could be impeding nitrogen use with lower pH. hot soil for a young plant is possible too. if that's the case, ignore the previous suggestion... if anything, the 6.0 would be helping in that context. too much light would be interveinal initially and spread quickly on top leaves. your cotyledons look fine. i don't think it is the case here. it could be evaporation increasing concentrations of available nutes in soil. So, think about recent behaviour, eliminate all the possibilities.. and if more than 1 is left, do the least invasive or not involving putting anything into your soil, first. always the safest way. can easily adjust from there

It could be yellowing slightly from those time release nutes. I would normally stay away from those. Next time find a good Supersoil or even Fox farms ocean forest which has enough nutes in it to last for 3-4 weeks with no feeding needed. Other than that man. It really is looking good!!