I grow the poinsettia plant because I can. The rain catches on the leaves like cabochons of perfect light. They glitter each leaf as the sun returns after a summer rain. They will be green, green, green until after the Autumnal Equinox.
Then as the days grow shorter, the sunlight changes its angle in the sky. Little by little, some of the leaves will turn red. As the leaf clusters turn, small yellow flowers grow within these bunches. The combination of these become what we call the Christmas flower.
It is important to allow the plant to create itself. All summer it will grow larger and larger. We must wait. The poinsettia is responsive to light and I would never think of forcing it. Mine stay outside until the nights get cold, like the end of October. This takes patience and a watchful eye. Let the plant be a miracle on its own. Enjoy the change. It will continue to grow.