Leaf colouration is caused by a variety of chemicals. According to the Royal Forestry Society, the main chemical in most leaves is chlorophyll. This gives leaves their green colour and helps the plant convert sunlight to energy. When leaves change colour in the autumn, it is because the chlorophyll breaks down and the other chemicals that give colour show through. These are anthocyanins, which are red, blue and purple, and carotenoids, which are yellow and orange. Plants' colours fade when there is an imbalance in these chemicals.
Sun and shade
Plant species react differently to varying levels of sunlight. A good example is the Japanese maple. Some varieties need full sun for their leaves to develop their colours, and others must be grown in a shady area to avoid the leaves becoming scorched. Check your plant's ideal growing conditions if its colour begins to fade. It may be that it needs a different growing site to bring out its full colour.
Sometimes leaves change colour because of a nutrient deficiency in the soil. Lack of nitrogen or phosphorous causes yellow leaves. Lack of potassium causes yellow or purple leaves. Magnesium deficiency causes yellowing between the veins of the leaves. Feed the plant regularly with liquid fertiliser according to the manufacturer's instructions. If growing in containers, scrape away any compost not filled with roots, and replace with fresh material.
Plants that are suffering from root problems will often show symptoms in their leaves. Phytophthora root rot is a disease that can affect trees, shrubs and plants. It causes leaves to yellow and die. In conifers, leaves turn dull green initially. There is no chemical treatment available to gardeners to treat this disease, but improving the drainage of the soil can help prevent its occurence. Waterlogged soils will cause most plants' roots to die.
Some plants naturally develop different leaf colours throughout their life cycle. One plant characteristic nurseries select for when breeding new varieties of plant is interesting foliage development, because gardeners can find this appealing. The Christmas berry, for example, has red-tinted young leaves that change to deep green as they mature. Your local nursery or garden centre will be able to tell you if your plant's leaf colour changes are part of its normal development.