Cauliflower is a flower. To keep the cauliflower head white, it is protected from the sun to prevent the development of cholorophyll which turns it green.
Peaks in the colder months.
Select clean, firm, compact heads that are white or creamy white. (The size of the head doesn’t affect its quality.) Any leaves that remain should be green and crisp. Avoid heads with major spots, speckles, bruises, or loose, open floret clusters. Some stores also sell packaged florets that have been trimmed off the head, and these, too, should be free of bruises or spots. Small leaves growing between the florets are not a sign of poor quality; just pull them out before you cook the cauliflower.
Store cauliflower in a plastic bag for up to a week. Keep the stem side up to prevent moisture from collecting.