Did you plant them from seeds or buy plants?

mine does the same, I think it is the heat.sun. Also the sweet potato vines that are normally a very dark purple, fade to a reddish purple. Same for some dark colored coleus. IME they regain their beautifiul hues when it cools down a bit

Zzackey.... I bought them as plants this spring. The basil was purple when I bought it, but as I harvest, the re-growth stays, well, green

vossner... Thx, I didn't think about the heat. That makes more sense. I've noticed my Black Pearl pepper plants aren't turning as dark this year either, so that's probably the culprit! I have no idea what zone I'm in, but I'm on the Alabama gulf coast, so heat & humidity have been extreme this year

The purple comes from the anthocyanin pigments present in the plant's genetic makeup. Google it if you care to know more about it, because hybrids that have been genetically modified are known to mutate and randomly revert back to their original colour. There have been dozens of studies (some of which are available online) on the instability of anthocyanins in basil alone.

I think studies have generally shown that lower light intensities can cause the production of the purple to slow, and that production stops totally in extreme heat. I think high phosphorus levels also play a part. But there are lots of other potential causes which you don't have control over.

Pretty sure also that once the plant reverts to green, any new leaves will be green. You can't make it go purple again.

Thank you Sophie, This is very interesting. I will definitely google it!

I haven't done anything to make my purple stuff (including that pretty ornamental pepper that has been mentioned) purple again. But when the sun/heat subsides, they HAVE regained their rich deep colors. Perhaps some plants don't, so I will be more observant to see which def. do. Even my purple crinum fades (like the basil) only to regain its beautiful, intense purple color later on.

Not basil but lookie here. I planted a rooted cutting which is perfectly purple. Plant to the left was planted 1 week ago and lost its purple to brutal all day sun. In another month I will post and you see all purple again. Still think it is sun effects. Strangely, when dark colored plants are in not enough sun, they also revert. That has happened to me also and has been corrected by moving to sunnier location or just not bothering with change.

I'm limited on where I can move them, as I live in an apartment complex, and have a covered balcony. I did place my container on the railing, and they get the maximum full morning sun, but nothing in the afternoon or evening. My black Pearl pepper plants don't seem to mind partial shade too much, as most of them are turning dark