If it was the cool weather, all the plants would be stressed at least somewhat.
Have any more plants come down with this affliction or is it the same ones? It's been a week since you first posted, and if it were a blight or viral infection it would be spreading to other plants. Take a close look at the plants nearest the sick ones.
If not, I'd think you do need to look at it as a soil problem. Btw, your County Extension service will do soil testing for you, but if it's the same as here, they charge more for it than to do it yourself. But they also might have an insight into the problem if it might be a local thing.
One more thing you could try related to the purple color you're seeing on new leaves is some Epsom Salts. It's magnesium in a soluble form, and tomatoes love it. About a tablespoon per gallon of water, dissolve it well and give each plant a gallon.
