Why are some plants’ leaves purple?

A plant might have purple leaves for any one of a number of reasons, but it’s usually because they’ve got a different colour pigment in their cells. The leaves might be rich in anthocyanin, a pigment that absorbs green and yellow light, making them appear reddish or purple. This protects new leaves from Sun damage and is also thought to protect tropical plants that grow in the shade of tall trees from fungal infections. When a plant that normally has green leaves – like marigolds and tomato plants – turns purple, however, it’s usually because of a phosphorus deficiency.