The calla lily's funnel-shaped flowers in white, red, pink or yellow is this plant's claim to fame. If your lily's flowers are changing colour, it could be a normal part of the plant's life cycle, or it could indicate an environmental problem.
When calla lilies enter the dormant period when active growth is over, the flowers will often change colour, turning green or brown, then wilt and fall over. If the plant is left dormant for at least 60 days, receiving no water, then repotted in fresh soil, it should begin regrowth with coloured flowers again.
Too little light will result in fading of lilies' flowers. If your calla lilies are indoors, move them to a room that receives brighter natural light, or supplement the sunlight by placing the lilies underneath a plant light. Make sure the room is warm enough and that no cold drafts affect the plant.
Conversely, calla lilies planted outdoors will fade and turn yellow if placed in too hot or bright a location. Move lilies to an area with full or partial shade to prevent burning and fading of flowers and foliage.