The purple colour is caused by a gene introduced from the snapdragon flower. This makes it a genetically modified (GM) plant. This being the case, you won't find it on sale in any supermarket.
Well,despite heathfield's explanation,I have seen an almost aubergine coloured tomato in Sainsburys.
They used to sell an organic tomato selection,which had the dark tomato,yellow,striped,unusual orange shaped and green(but ripe).
They looked really great in salad,but sadly they haven't had them now for about 2 years.