They're tender annuals, I guess, so I'm amazed anyone had small plants to sell to you!
You could try overwintering them indoors in a just warm room, with the aim to plant them out after the frosts next year.
My plants have not done very well - cold, wet August put them off, I guess. Depending on the variety, they may stay green when ripe - or go purple. They are not tomatoes, but more like Cape Gooseberries.