Purple cauliflower contains anthocyanins.
Anthocyanin is any of a group of water soluble, flavonoid pigments (glycosides) occurring in solution in vacuoles in flowers, fruits, stems, and leaves. They change color depending on the acidity of the solution.
Anthocyanins are responsible for most red, purple, and blue colors in plants, especially in flowers, which serve to attract insects and other animals for purposes of pollination and seed dispersal. They also contribute to the autumn coloring of leaves, and tint young shoots and buds in spring. These colors may be modified by other colorless or yellow flavonoids.