You are growing white cauliflower and tying up the heads to blanch them, right. If you are in an area with strong sun, it will cause the heads to color. From The U of Wisconsin: "Some cauliflower varieties have a genetic propensity to having a higher concentration of red, purple, or blue pigments. This is the same harmless, water soluble pigment found in eggplant, red cabbage, berries, plums and grapes. In other types of cauliflower the colorless or white pigments will predominate. Purpling can develop in white varieties of cauliflower if the heads are exposed to light while developing. Usually, the leaves are tied over the heads.

If the cauliflower has a lot of purpling it is probably best to use it raw for relishes or salads. Heat may induce a color change from purple to gray or slate blue--especially if the water is hard or had an alkaline pH. If you prefer to have cooked cauliflower add a little vinegar or cream of tartar (tartaric acid) to the water."

Thanks, I am late in getting the leaves secured with the rubber bands this year. Thanks for your response.

I find that a cloths pin is an easy fix, just clip 3 leaves for shade.