Lily plant leaves should be bright green. They may be a deep, glossy green, as with the peace lily, or a lighter, vivid green, as with Oriental lilies. Yellow colouring in the leaves often indicates that something is wrong with your plant.
Lilies grown as houseplants, such as the peace lily, need a period of dormancy each year. During this time, the leaves turn yellow, the plant produces no new flower buds, and the lily can look bedraggled, like it is dying. This is natural and can happen during the winter or when you repot a houseplant lily. If you suspect your indoor lily is trying to go dormant, reduce watering for six to eight weeks, at which point the plant should resume typical growth.
Too much or too little of this natural resource can cause leaf yellowing. If you notice yellow leaves on an indoor or outdoor lily, touch the soil. If the soil feels dry and dusty, you're probably not watering the plant enough. If the soil feels slushy or boggy, you're giving the lily far too much water. In the future, water plants when they feel dry to the touch; saturate the soil, then allow it to dry out but not get parched.
Lily plants that receive too much wind, whether outdoors or indoors in a draughty location, can develop yellow leaves from the stress. Excess salt -- from water with a high salt content or cow manure, which is salty -- also can yellow leaves. Try to identify the cause of the salt saturation, and remove it to restore your lilies to health. Also, old lily leaves turn yellow when they are dying. If only a few leaves on your plant are yellow, it could be because of age. Clip off the yellowed leaves to enjoy a healthy-looking plant once more.
Make sure that your indoor lily plants grow in containers with drainage holes and that no water sits in the saucer below your container. Lily leaves can turn yellow when the plant sits in water. Always check the soil's moisture content before you water, so you know when the plant needs it.