Plants, trees, and cacti all have their breaking points when it comes to handling what Mother Nature throws at them. They have mechanisms in place to help them react to weather changes as we have the capability to react to either cold or hot temperatures.
Have you noticed that the normal green-hued prickly pear cacti are all shades of purple at the moment? What you are seeing is this plant’s adaptation at work.
Cacti react to environmental stresses by turning all sorts of reddish to purple colors. When they turn these colors they are reacting to the cold and dry conditions of their environment. So without even seeing a temperature or rain gauge, you can tell that the region is experiencing extremes of cold, as well as lack of rain or snow. When the rains return hopefully they will turn back to their green coloration.
I do want to note some cactus species are purple by nature.
All living things have their limits. It is more evidently seen by the cacti turning purple to deal with cold temperatures and lack of moisture.
Forgotten Winds 