My advice, not only to you, but to every new grower I've ever tried to help, is to learn/research how a cannabis plant grows. Learn the stages.. What the leave colors mean at the different stages. And never stop learning/researching.
You're doing the right thing by noticing the difference between the plants.. The color, the size, ect. That's an excellent habit to form. But also keep in mind that every, and I mean every, plant is different. One may grow slower, or be sensitive to light.. One might be a heavy feeder, ect.
Your plants are fine right now. Often times seedlings will have light or bright yellow new growth as they start to photosynthesize and make more chlorophyll. The color should improve over the next few days. The plant doesn't need much of anything at this age. It can get food from its cotyledons if need be. No need to add or do anything extra. And like others have said.. Practice patients.. You'll probably be growing for the rest of your life.. So waiting a few days before you take action on a plant, won't hurt in the long run.
