The Art of Hanakami, or Flower-Petal Folding

Beautiful! But those are not the petals of bougainvillea. The actual flowers are white and much too small to be folded.

I can assure you that they are definitely bougainvillea flowers that I folded. In fact there appears to be quite a few varieties and they do possess different colours: https://en.wikipedia.org/wiki/Bougainvillea

You are right in that they are difficult to fold, but not because of the size of petals (I fold models no bigger than from a 1cm x 1cm square cut from the petal), but because of the curvature of the petals that makes it difficult to press and work with.