Lack of Nitrogen & Iron

Lack of nitrogen:
Use liquid seaweed with iron. (Alaska Fish Fertilizer, Neptune's Havest, many other brands)

Neptune's uses a hydrolyzed method for more amino acids and enzymes, a little higher in phosphorous. Others are balanced with other nutrients, like the Ladybug brand that includes liquid humus, molasses, and compost tea.

Lack of iron:
If a plant is yellowed and chlorotic, it's in distress, which can lead to insect and disease problems. Some of the liquid seaweed products contain iron.

It could also be a lack of magnesium in the soil. You can add magnesium sulphate (Epsom salts). You don't want to over-use it, since it's a trace element and not much is needed. If it works the first time, don't use it again for at least a year, because you'll get out of balance with the calcium in the soil, and it's the relationship between the two that makes it work. What happens is when this is applied other nutrients are picked up out of the soil and moved throughout the plant thus greening it up very nicely.

Copperas is also a good one for acid-loving plants. It's 11% sulfur and 19% iron, and does a good job on plants that need a more acidic condition. This does a good job on plants that need a more acidic condition.

Greensand has been the classic source of iron. Some products also include some potassium so you get that nice combination.

You can use these as a drench or a foliar feed for quick greening up.