Bougainvillea flowers drying out.

Question

Plant in soil doing well then this happened.

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Answer 1

Yep! Those are "flowers", and you have a yellow form.

The flowers are actually modified leaf bracts, and some are yellow. I see reproductive organs, so I can safely say that those are flowers.

This article will help you to care for these plants: https://www.gardeningknowhow.com/ornamental/vines/bougainvillea/bougainvillea-garden-care.htm