I've talked about this with some others before. A guy by the name of David Hershey has written specifically on this topic before, and some of what is below is a direct quote. You can read his qualifications here:
http://www.angelfire.com/ab6/hershey/bio.htm
In the fall when leaves senesce, they often produce more anthocyanins, which often function to protect the leaves from high light or ultraviolet light.
A similar mechanism might be operating for a nitrogen deficient leaf which is also exporting large amounts of nitrogen. Fall leaf senescence is similar to a leaf nitrogen deficiency because the chlorophyll is no longer being synthesized and mineral nutrients, such as nitrogen, are being exported from the leaf. An older leaf dying from lack of nitrogen is undergoing a similar process. Chlorophyll cannot be synthesized due to lack of nitrogen so the leaf becomes chlorotic, meaning it turns yellow or white as chlorophyll is degraded and the yellow carotenoids are revealed. The nitrogen in the leaf is transported to growing points such as shoot tips, which have a higher demand for nitrogen.
Anthocyanin formation occurs with some other deficiencies, particularly phosphorus and sulfur. Tomato plants often show purpling of their leaf undersides as a phosphorus deficiency symptom. Remember that one thing that may complicate the appearance of leaf anthocyanins in nutrient deficient leaves is whether they are being synthesized in response to a nutrient deficiency or if preexisting anthocyanins are simply becoming visible because of the loss of chlorophyll from the leaves."
So, you are seeing a purple color at the end of the flowering period because the plant is reaching end of life and is no longer producing chlorophyll.
