Cauliflower curds, the edible white part, can turn yellow if they are exposed to sunlight.
To stop this gather all the outer leaves and tie them together by either using garden twine or, even better, use an elastic band as they can expand themselves as the plant grows bigger. The aim is to keep sunlight off the curds. If you use twine, check it every few weeks to make sure it isn’t cutting into the outer leaves.
Harvest when at the required size and before the curds ‘flower’ which is when the tightly packed curds start to separate.