Might be just older leaves.

Mine did that last year while in a container - could be for different reasons, but I don't think it's a disease. I ripped out part of last year's plant from the garden and now it's growing in a new container.

Could be growing conditions. My oregano, lemon balm, peppermint, etc. have different leaf colors, often reddish, at the end of the growing season, heading into fall. Could be maybe your temperature or other conditions that are stressing the plant?

Or sometimes new leaves in the spring have other coloration as well.

FataMorgana

Ummm... I have two pots sitting next to each other. One is normal, but the one in the picture now is in a very bad condition. Even the new shoots are either yellow or red. And I noticed that the healthy pot starts to develop the same symptom.

They both receive full sun, with fairly warm temperature in my garden, with a potting mix of soil, charcoal, and manure. There is an earthworm living in the healthier one. They are living in a row next to rosemary, peppermint, and lemon basil, all doing very well.

There is no insect found in the plant, no aphids, ants, scales, nothing.

Ok, it's fairly simple. They are very healthy now. It is just too much heat. The healthy one was shadowed by a tree. The bad one is just slightly out of the shadow. With the change of season the shadow is moving away. I move both to another location, and all is well.

I smell pizza!