I've been reading online about the tips of cacti going yellow due to excessive sun, but it seems that every other article indicates it could be not enough sun, too much water, not enough, water, etc., so I'm wondering if anyone has any insight as to why these 2 might be a bit yellow (hoping the photos upload appropriately - have been having issues with this).

This first plant is in a windowsill - it gets probably 3-4 hours of direct sunlight a day in New Mexico (about 6,000 feet, so strong sun). I planted it about a month ago after buying from a pretty reputable nursery in town.




The second is in a room that gets very bright, but is not in direct sunlight. It was also replanted around the same time.



I've been very careful to not overwater, so the soil is pretty dry at this point. I wanted to give the roots a chance to recover after repotting.

I think it is sometimes a combination of heat and then periods of cold temps. I get that with my oldest Eve's needle. Thankfully it has endured and survived the fluctuations here. And I guess the age of the plant too also contributes to that aspect.

This plant lives outdoors year round, so it gets all the temperature and.seasonal light changes:








A younger batch of cuttings taken from that mother plant many years ago has grown good and I guess since it is much younger it still stays solid green and ably makes it leaves:

I'm guessing it has to do with insufficient light. From the photos it looks to me more like pale green instead of yellow, which would sort of suggest low light, especially if it were combined with narrowing stems (which I'm seeing in the first pic) and lengthening internodes (which I'm not really seeing anywhere). Those are the classic indicators of etiolation, so you know what to look for down the road. The internode is the distance between successive areoles along a stem.

Another thing relating to the blue color of the first plant, at least toward the base. Blue cacti are often bluer in strong light, and I usually try to dial up the sun to enjoy that aspect. Some of them seem to be needy about light for good health as well. Depending on the plant, they may actually turn more of a blue-green or pale green when they are receiving a lot of protection. What I'm guessing is the first plant changed location when it was roughly half as tall as it is now, from a bright place to a dimmer place. That transition may have taken place between the grower and the seller, because your plant has put on very little new growth in a month's time even under the best of conditions. This is probably a historic effect, most likely nothing to do with your care thus far. Was that blue cactus actually all blue when you got it, or did you just recently notice it is two different colors? Subtle changes in color are not always obvious to us when we aren't paying attention.

Not knowing the ID of your blue cactus, I can't really speculate much more than that.

As for watering, it's good that you're paying attention and exercising discipline, and it's good to give the roots a time out after repotting. The ideal watering cycle (from my viewpoint) should go from saturated (but not supersaturated, ie. sitting in a pool of water) to dry or almost dry (at depth, not just at the surface) in a fairly reliable way, life and other matters permitting. The lower the light, the less frequent the water. If a plant is not seeing the sun for hours a day year round (weather permitting) then maybe cut its frequency in half during dark periods, until you are able to get a better handle on the situation through observation.

@tarev - thanks for your reply, and my plant is pretty new to my house, so not tons of fluctuation here, but certainly possibly before I got it. Not sure how old this is, so maybe if it was alive last winter?

@Baja_Costero - I had noticed the narrowing stem on the first pic as well, which is what I think had me concerned in the first place.

Looking back through my photos (I try to keep records so I can compare), I'm actually thinking I got this one about a week ago (I've been getting a lot recently), so I'm pretty sure it doesn't have to do with conditions here. It's getting about the best light in the house, so I guess only so much I can do with that one. (And in terms of keeping them outside, it rains way too much here during monsoon - it would rot in a week or two with the downpours we have).

Here's a picture of when I first got it a week ago. Doesn't really seem like much difference.

Yeah it should look great in that sunny windowsill, just give it time.