What Caused Parts of This Cauliflower Head to Purple?
This cauliflower has some purpling/reddening caused by exposure to sunlight. It is important to have enough healthy leaves on the plant to cover the head as it develops. If for any reason the leaves do not cover the head (poor growth, foliar disease, wilting from heat, etc.), it will discolor, and marketability will be harmed.
For more information on growing cauliflower, see HGIC 1326, Cauliflower.