From Wikipedia:

White:
White cauliflower is the most common color of cauliflower.

Orange:
Orange cauliflower (B. oleracea L. var. botrytis) contains 25% more vitamin A than white varieties. This trait came from a natural mutant found in a cauliflower field in Canada. Cultivars include ‘Cheddar’ and ‘Orange Bouquet’.

Green:
Green cauliflower, of the B. oleracea botrytis group, is sometimes called broccoflower. It is available both with the normal curd shape and a variant spiky curd called Romanesco broccoli. Both types have been commercially available in the U.S. and Europe since the early 1990s. Green-curded varieties include ‘Alverda’, ‘Green Goddess’ and ‘Vorda’. Romanesco varieties include ‘Minaret’ and ‘Veronica’.

Purple:
The purple color in this cauliflower is caused by the presence of the antioxidant group anthocyanins, which can also be found in red cabbage and red wine. Varieties include ‘Graffiti’ and ‘Purple Cape’. In Great Britain and southern Italy, a broccoli with tiny flower buds is sold as a vegetable under the name “purple cauliflower”. It is not the same as standard cauliflower with a purple curd.