Why is a purple leaf, purple in color?
There are three main groups of plant pigments: chlorophylls, carotenoids, and anthocyanins. As you already know, chlorophyll is green. Carotenoids are orangish-yellow, and anthocyanins are red or blue depending on pH. A purple plant appears purple because it has high levels of anthocyanins and a fairly neutral pH. Red anthocyanins absorb UV light and protect the plants tissues from damage. The undersides of many rainforest floor plants are purple to reflect light absorbed by the top of the leaf (because green light absorbs red and blue light) back up to increase absorption by auxillary pigments like carotenoids. Every plant has chlorophyll, but some plants just have so many anthocyanins that their pigments overwhelm the green color, just as chlorophyll usually overwhelms the other pigments in green leaves except during fall when the chlorophyll is withdrawn.
