You may have noticed some varieties of cauliflower are not their normal colour. They may be showing hues of brown, pink, purple and yellow. And when cooked, it can go grey or blue.
Cauliflower is highly heat-sensitive, and certain varieties are more sensitive than others. The recent run of hot weather with lots of sun and few clouds has caused anthocyanin pigments, a harmless, natural antioxidant, to discolour the plants.
It is completely safe to eat.