All the leaves are turning wine from the edge inwards. Not sure what is causing this, and after a long search I found nothing similar. Anybody?
One cause of purple leaf discoloration is phosphorous deficiency. This can be aggravated by low temperatures, which might explain why it has started recently (depending where in the world you are gardening, of course).
The purple colour is the result of the plant being unable to producing enough green chlorophyll. Without chlorophyll, many plants would be red or brown, not green.
Try a high-phosphorous fertilizer. There are various commercial brands sold as specifically for flowering plants, or plants like tomatoes which have high nutrient requirements.
Soil acidity can also inhibit phosphorous take-up by plants, but unless you want to do a soil test, just try an appropriate brand of fertilizer first.
The only time I've seen this coloration on the edge of the leaves of Tagetes has been when its either too cold (esepcially at night) or the plant is suffering, or has suffered, some drought. You don't say if your plants have been in or outdoors, but overnight temperatures might be a factor, as it was early spring when you bought them, otherwise, possibly insufficient water. That can be caused because they've run out of root room when in small pots, or simply because they're either not watered often enough or with enough water when they are given some.