Cabbage root fly. Are the plants also stunted and not going to grow to any size? This is typical symptom of attack. If you lift one of the affected plants you will find that it has very little root left and you may even see the small white grubs on it.

thank you for your reply, i have just pulled some up i can't see anything on them at all the roots look good could there me another reason or could you tell me what i can do to stop this

Can I just add that there is a type of cauliflower that is quite a vivid lilac colour. I have bought it in the past. I think you have the seeds for that kind even if the packet doesn't suggest it. It does look quite peculiar but tastes just the same.

Also unless you pull the leaves over the curd then it will go a pinky/mauve shade.