I have two poinsettia's that I have somehow kept alive for a year. They were green all summer. I wondered if they would turn red. This is the smaller of the two and it has red leaf buds all over, the larger one is still all green. I googled about the red color and here's the information:
The Poinsettia is a light sensitive plant. When you deprive the plant in its full leafing stage of light, the only chlorophyll used to turn the leaves green cannot be produced. As a result of this total darkness and lack of light, the only color that will be produced is red. This is called photoperiodism.
I guess with the sun setting at 4:45 and rising at 7:15 it is getting enough darkness now.
When I was in college I worked it the greenhouse and we pulled shade cloths over them to shorten the days so they wouldn’t produce that love red color.
Great details!