Stress colors don't always mean the plant is under stress in the sense it needs different care. Sometimes it's the result of natural factors like day length or temperature. Here are a couple of photos of the same plant at different times of the year. The purple color occurs in the fall when the days are shorter and cooler. Then in the warm months of late spring and summer, it gradually turns green. This is normal for this particular cactus and it reliably goes through this change every year. As it has gotten older, it seems more reluctant to shed the purple hue in favor of the green but that may still be a response to the conditions. This is Echinocereus subinermis at different times of the year.
