If you want your garden to produce snowy white heads of cauliflower, you have to somehow keep sunlight from hitting the growing heads. There are varieties of cauliflower that are touted to be "self-blanching" which means the leaves grow in a manner that they naturally curl over the top of the head to form a protective covering.
I've tried that . . . without success. The heads were not totally covered and developed the undesirable purple-ish/brown off-color so I went back to the method used most often by home gardeners. And that is to physically tie the leaves over the heads of cauliflower.
In correspondence, Rain over at Rain's Garden commented that she would like to see a picture of the tied up heads to better understand this method.
Here's my remaining bed of cauliflower plants. I didn't have enough started plants to fill the whole bed so put some marigolds and geraniums at each end. Looks pretty much a mad jumble, doesn't it.
Looking closer, here's a shot of a single head.
A cauliflower plant will put out many leaves radiating out from the base of the plant. The head will start to form in the center of these leaves. When the head is about 2" across, it's time to cover it to protect direct sunlight from hitting the developing head. In the picture, I've pulled all but the very bottom layer of leaves up over the head and tied them with string.
You may notice (dang and drat) that the leaves have holes in them. Yep, I've been invaded by what I'm pretty sure are flea beetles who love to feast on vegetable plant leaves. They're doing the worst damage on my brassicas with some other veggies being spared. Papa Pea sprayed with an organic solution which left a white-ish residue on the leaves, but does no harm . . . except to the flea beetles, if all goes well.
Hoping this gives you a clearer picture, Rain, of how cauliflower leaves are tied up over the developing heads.
As an ending note: Death to flea beetles!