Here's the image URL again:

Here is a link that might be useful: Images of Tomatoes on Flickr

Stress from low water, perhaps from dry soil, windy conditions, low humidity and/or a heat wave.

purple is usually a sign of a phosphorus deficiency.

I will say that the are in awfully small containers and be carfull not to over water and watch drainage in that style of pot. Water like to sit in the bottom of them.

Purpling indicates things other than phosphorus deficiency.

In this instance, the accompanying dried (tan-colored ) cells indicates a water shortage in the top of the plant.

As suggested earlier, that can have multiple origins.