Why are my viola leaves turning brown/purple ish?

[rumbypumby]

I have them in 3 different spots in the garden, varying sun and shady areas. All receive similar amounts of sunlight and water. All are turning a purple brown colour at the leaves.

Any suggestions?

Are they spring-flowering ones you bought earlier in the year that have already flowered?

They are still flowering but not as quickly as usual and not looking great. They flowered until late summer last year 🤷🏻‍♀️

Some plants develop a red colour as a defence against too much sun