Usually, if you plant is yellowing throughout, I would go with high nitrogen like 20-10-10 or 10-5-5 to be safe. But if all looks well, 10-20-20 is the standard in fertilizers. You can pick up some cheap, yet decent, ferts at k-mart or walmart, or any home and garden stores like Home Depot. When buying higher nitrogen ferts to darken up ur leaves, check out some of the ferts labeled grass fertilizer. They are usually hi nitrogen and are inexpensive. A lot of people like to buy hi nitrogen ferts, but honestly, if your leaves are nice and green 10-20-20 is excellent. Been using it on my family farm for decades. Farming is our family business.