Why is my cauliflower turning yellow?

White cauliflower curds turn yellow when exposed to sunlight — the fix is blanching: tie outer leaves over the developing curd with a rubber band as soon as it appears (walnut-sized). Delayed blanching in India's bright winter sun causes rapid yellowing within 3–4 days. Heat above 22°C also yellows and opens the curd. Boron deficiency causes brown, hollow curds with a bitter taste — apply borax solution (1 g per litre) as a foliar spray.