Gosh, pretty sad looking plants! When did you last fertilize?
We really can't help much without knowing what kind of potting mix you use, if the containers drain well, when you last re-potted, and stuff like that.

At first glance, however, the plants appear to be very nutrient deficient.

Thanks rhizo. I never thought it could be a nutrient deficiency! I'd actually be happy if it was so I can remedy it.

The mint was repotted last summer, so less than a year ago. I haven't fertilised much to be honest... Would a typical garden centre fertiliser for vegetables do? My compost isn't quite ready for use.

I actually have never fertilised my lavender as I read that they don't do well if the soil is too rich and prefer poor soil!! It was repotted llate last summer and is sitting in some sandy mix. Could I have been so wrong??

Thanks again!

Let's remember that your plants are in containers. They need to be treated as such; they have NO way of obtaining any soluble nutrients unless you provide it for them. The worlds of outdoor in situ planting and container planting are as different as night and day.

I'd purchase a regular houseplant fertilizer, one that you can mix with water and apply it that way. It will go to work MUCH faster.

Thanks so much! I will do that this evening.

Some of my herbs, like lemon balm and thyme, turn purple when they get too cold - like in early spring and late fall. I would say that temperatures are a factor as well.

FataMorgana

Thanks all!

I've fertilised the mint and lavender last night. So let's see. Agree that weather might be a factor as they seemed to get worse after I put them out.

Reddish/purplish leaves are a sign of phosphorous deficiency, something that is very common in dry soil and potted plant culture (nutrients need water to get into the plants). Do give a good well rounded fertilizer with micronutrients and stay away from the typical NPK ones and make sure they are not drying out too much or too frequently.

A deficiency of phosphorus (P) CAN BE manifest in purple foliage because P is required to make ATP and ATP is needed to turn sugars to starch and to load sugar into phloem for transport. No P - no ATP - no move sugars/starches - anthocyanin (purple pigment) builds up - plant turns purple. But purple foliage is far from an absolute indication of P deficiency.

The most frequent cause of a P deficiency is cool night temps and sunny days, which is why so many plants planted out too early turn purple. This is an environmentally induced deficiency - not an actual deficiency and should remedy itself as nights warm.
A P deficiency is not the only suspect cause of purple leaves. Nearly any environmental condition that puts the brakes on growth and the accompanying use of sugars, but does not limit sugar production (photosynthesis) can cause anthocyanin buildup and purple leaves.

If only the outer edges of the leaves are purple, it may be a K or Mg deficiency. If the center of the leaves are also purple, it could be too much Ca in the soil or the result of too much water in the soil blocking uptake of P and Mg.

Do not apply a fertilizer with the middle number higher than either of the other two numbers (N & K) to container plants. There is never a need for that much P (relative to N) in container culture unless you're supplying only N in another form. Plants use about 6 times more N than P and the massive doses of P in 'bloom-booster fertilizers' cannot do any good.

Other causes of purple foliage include
Drought response
A high level of soluble salts in the soil
Nutrient deficiencies/toxicities other than P
Anoxia/saturated soil/compaction
Herbicide injury
Exposure to bacterial/fungal pathogens

Al