https://letmegooglethat.com/?q=protect+cauliflower+sun

I don't think there is any reason to wrap up purple cauliflower, wrapping is meant to keep white cauliflower light colored. You want the deep purple color.

Cauliflower likes mild weather, and I think in your climate you want it to grow all Winter, so shade is not ideal, since you want it heading before Summer.

That's a fair point. In fact, sunlight exaggerates the purple color, I believe. Anthocyanin. Low temperatures, as well as lots of sunlight help make it. Anthocyanins are also what makes berries purple.

Back when I used to grow white Cali heads a simple clothspin worked well to clip 2 leaves over the top crown for good shading.

“Cauliflower is the hardest of the brassicas to grow.”

Yes, worrying about sunburn is way down the line. If you get to that point with them you will already have done very well indeed.

I'm thinking that most of the growth will happen in early spring when the sun starts hitting that bed again. We shall see!

Purple cauliflower or orange cauliflower are as easy to grow as the white. Those of us with hot summers must be constantly cognizant of planting times and DTM of variety. Purples tend to have a longer DTM and sometimes get caught in a heat wave. On the other hand no need to wrap or us other methods of blanching.

Been gardening for over 50 years in a number of states and I've NEVER had luck with cauliflower. Quit trying some years ago.

Well, I'm in California, and in town, so not much frost, but some cold temps coming up! Unfortunately that bed doesn't get much sun during the winter. It will early spring, but we shall see if it's enough to get something before spring planting time for tomatoes etc!