These pigments normally range in colour from yellow and orange to red. It's easy to remember the name of these orange pigments once you know that they were first identified as the colouring agents in carrots. Yellow carotenoid pigments are present in most leaves during summer, but because chlorophyll is also present at this time, their yellow colour is concealed. When a plant suffers from a lack of nitrogen, however, the level of chlorophyll decreases, and the yellow colour of the carotenoid pigments become more obvious. Very often, when a plant turns yellow, it is because the chlorophyll level has decreased, and the carotenoid pigments have simply become more obvious.
Most healthy plants have a high level of chlorophyll, so they appear bright green.
As temperatures begin to decline, and days begin to shorten in late summer, the chlorophyll in the leaves begins to break down. The same conditions which lead to chlorophyll breakdown, also lead to increases in anthocyanin and carotenoid pigments. The leaves are therefore losing the green colour as the yellow and red colours are increasing. The most dramatic show of colours is therefore seen on those autumns where there is a well-defined shift away from warm summer temperatures, to cool autumn temperatures. These conditions speed the breakdown of chlorophyll, and increase the productions of anthocyanins and carotenoids, to produce bright clear colours.