Are you leaving them in strong sunshine uncovered?

Thanks. They have bee n in the recent sun light,does this make a difference?

Yes it can spoil head development - fold the leaves over the heads, but I dont think this would discolour them? Downy mildew sends the heads brown not purple. A photo would help?

there is a type of cauliflower available that has a purple heart. I bought one in Morrison's not so long ago. Although it tasted fine, the colour was quite off-putting. Just check the details of your seed packet to ensure that this isn't what you are growing before you do anything drastic.