Santa Rita Prickly Pear.
Santa Rita cactus pads can change color in response to temperature and sunlight. This is due to a pigment called betalain.

Explanation
Environmental stress: Cacti can turn purple as a way to cope with environmental stress.
Seasonal color change: Cacti are more likely to turn purple during cooler winters.
Water: The less water a Santa Rita cactus gets, the more purple it becomes.