In Temperate regions, two distinct processes are involved when leaves change colour in the Fall. Yellow and orange colours reflect carotenoid pigments that are already present in the leaves. Reds reflect something different – anthocyanins that are produced as part of the process of senescence.
The red colours produced by pointsettias represent something different. In this case, leaves change colour in order to advertise the (relatively inconspicuous) flowers to pollinators. Pomar and Ros Barceló asked the question of whether these leaves are photosynthetically active – after all, that’s a lot of leaf area to sacrifice just for reproduction. Their findings?
Both red and green poinsettia leaves are able to use photons efficiently to perform photosynthesis in the range of irradiances common on the Earth’s surface.
At lower light levels green leaves are better at photosynthesis than red ones, but at the level of sunlight you’d expect to see on a sunny day, the differences were very small. However, red leaves aren’t very good at dissipating excess energy. However, they compensate for this by having the red anthocyanins which absorb some of the incoming radiation.
Pomar, F., Ros Barceló, A. (2007). Are red leaves photosynthetically active?. Biologia Plantarum, 51(4), 799-800. DOI: 10.1007/s10535-007-0164-z