Welcome to GoY.

It sounds as if it is missing some nutrients such as magnesium. Is it in a pot or in the ground? How long have you had it? Either way it needs a feed, at this stage I would give it a teaspoon of Epsom salts diluted in 1/2 gallon of water. And then see if it greens up.

Has it been sitting in water for any length of time if yes it needs lifting and repotting.

It also helps to know exactly how the leaves are turning yellow:
Older leaves turning bright yellow and dropping off, and small twigs dying, often mean nitrogen deficiency--rare in the ground, but common in pots. Such could also mean drought damage or a drainage issue.
Older leaves with yellow and brown tips are usually caused by excess sodium.
Older leaves with yellow and brown edges are usually a sign of magnesium deficiency.
If the younger leaves are light grean or pale yellow with greener veins, a condition called chlorosis, it is usually a sign of iron, zinc, or manganese deficiency, frequently caused by very alkaline soil. Chlorosis often occurs in salty or poorly drained soils.

Tugbrethil, you really are a fount of info. Thats so helpful! I always think yellowing is as a result of too much waterlogging, but you have educated me! Well, if I could remember it all that is!