Looks like you need a higher level of nitrogen in your soil, but wait! if your not using quality Nutrients you will want to wait 3-4 week before doing so for 2 reasons
1. the plants are still young and you could kill your plants through Nute burn.
2. your premix soil already contains a cirtain level of Nutes in your soil that you will deffinatly want to washout before you use the good stuff.
Spend the money on quality products it will make all the difference in every aspect of your grow, DO NOT!! use Miraclegrow EVER!!! but proper lighting should be your main concern.
Tip: Read the grow guide, bookmark it, make it your new bible! Liveit,Love it & smoke it! GOOD LUCK.