The cole crops are favorites of bugs, especially little green worms and aphids. Cabbage, broccoli, cauliflower, kohlrabi, kale, and turnips, are all in the cole family. To be successful, with these crops you must
address the bugs.
I can't say for sure about the color, but suggest it is because of stress, and possibly the stress comes from the aphids.
It is sometimes desirable to blanch the cauliflower heads. You can do that by tieing the leaves up over the head, or by putting a brown paper bag over the head and tieing it. Then you get white heads.
In any case, you need to get the bugs off it. They will ruin it. Try a mild soap solution in a spray bottle, spray the bugs and wait 5 minutes then rinse with water. A soap solution kills aphids very quickly. It has no residual effect though, so you have to keep an eye out for reinfestation.
I use Dawn dish washing liquid. A couple of teaspoons in a pint of water. Now as soon as I say that, someone will come along screaming that Dawn is a detergent and you can't use it ........... Oh well, hide and watch.
Gardening at 5000 feet elevation, zone 4/5 Northern Utah, Frost free from May 25 to September 8 +/-