I'm hoping to save a fairy castle cactus I found at my parents' house. It has some odd discoloration as seen in the photos, brownish with some areas leaning towards yellow and some more red. My parents had it for about a year and hadn't changed it from the pot/soil it came in from Costco. About 2 months ago I repotted it in around 50/50 Miracle Gro brand cactus potting mix & perlite. I watered it once about a month ago and haven't since then, and about two weeks ago I moved it from a very sunny SW facing window to a corner with NW & NE facing windows where it gets relatively bright indirect light. The color hasn't really gotten much noticeably better or worse, so I think it'll need more drastic action, but I don't really know what that action should be or what the problem is exactly.

My only guesses at this point are either root rot or sunburn. The discolored areas don't feel particularly squishy, and I've been very careful not to overwater since it's been in my care (if anything it might be underwatered), but could it be leftover from before? Or could it be sunburn leftover from when it was in the SW window? I'm not sure how long it takes to recover from sunburn, or if they can even recover at all. Or could the issue be something else that I haven't considered? Thanks in advance for your advice!

I haven't grown the plant, but what I see reminds me of surface stress color. Those can be a response to seasonal temperature and light changes. Of course they can also be caused by moisture levels in the soil. Too much or too little.
It doesn't sound like that should be the cause since it's been under your care. Certainly changes in sun can cause it, but it really doesn't look sunburned to me - its too even on all sides. If it's been inside all along, that's not likely. I'd guess in the long run the plant will be better off with as much direct light coming in from a window as you can provide.

For example, I have an Echinocereus subinermis that is currently in the process of turning purple in response to the shorter days and cooler temps. It does it every year. It stayed cool and damp for a long time this past spring, so the green stage is shortened this year. Purple seems to be its winter coat color.

Just some thoughts. Perhaps someone else will have other, better ideas.

Thanks for your advice, needrain! Hopefully it is just seasonal stress, I'll hold off on any major changes and see if it goes away in the spring.

I agree with Donald. I don't see any actual burning on your Fairy Castle Cactus, just some really nice stress coloration.

Hi all. I found garden.org just now when googling for an answer to this very same question! Very excited to have found this resource!

My Fairy Castle cactus has similarly started a color change, after I moved from an apartment where it was in a NW facing window getting consistent but indirect sun (including late-day bounce off the glass building across the street) to an apartment with two huge south-facing windows that get all-day BRIGHT, DIRECT sun. When I moved 3 weeks ago the little fella was a robust green (Photo #1), but in the last 7-10 days has begun turning a sort of sickly yellow-brown (photo #2). It is not mushy, and I water it only about once a week, when soil is very dry. So I'm hoping this might be a stress reaction to the sudden change in sun?

If so, would you all advise as above to keep it where it gets all that sun? Or should I remove it to a spot with less direct light for it to 'recover'?

-Corey

Yes, it sounds like sun stress. Keep it where it is unless there is some further color change, just rotate it regularly so that a different part of the plant is in the shade.