This has been going on for a while (about a year), one of my cacti has turned purple. I've just replanted it after its pot fell and broke (no, it did not spend the year on the ground). What is the cause of this and what should I do about it?
I put the cacti out on the open balcony in the summer and put them on the windowsill for the rest of the year, so the usual temperature they are exposed to is 20-35C (as low as 10C during the night). I water them twice a month during the summer and once a month otherwise with tap water, about 100 ml per plant. The height of the cactus is about 6.5 cm. The cactus in question spends most of its time very close to my other cacti (~10 cm), and some of them don't look too hot, pictures below.
Cactus in question about to be replanted, click to open full image
Section of the offspring, the flesh is green with a white center and has no smell, click to open full image
Neighbouring plants showing possibly related symptoms (The other side of the one on the left is perfectly green), click to open full image