One of my plants is a much darker green then the o...

Hey the little plant is a bit heavy on nitrogen but not at a toxicity level yet. The leaf tips will start to brown and burn if she gets much more. The other one is a faster grower so processing a little more hence the lighter color. I noticed that you added nutrients to the soil. That fox farm soil is a high quality soil that is complete. It has everything the plant need for nutrients to last around 3 or 4 weeks. Adding nutrients is what caused this. With that being said she will be hitting her growth spurt any day now and start sucking up that nitrogen. You dont have anything to worry about just give plain ph adjusted water until you see signs of them being hungry. With the nutrients you added you might make it to flower before you need nutrients and when that time comes throw her some bloom food. And more is not always better with nutrients. If she gets overloaded then she wont be able to absorb any nutrients so slow and steady wins the race. Good luck with them