Bougainvillea, like our native and ornamental Poinsettia, uses bracts or leaves to advertise location of tiny flowers which have petals and sepals.
Each Bougainvillea 'flower' has three small whitish yellow tubular flowers. They are tubular with a narrow opening and only a bee or butterfly wit a long thin tongue can access the nectar. On the way in or out the tongue picks up pollen.
It would be interesting to observe whether the different colors of Bougainvillea brats are equally attractive to pollinators.