Oh my goodness! All these green tomatoes and they are not ripe, nearing the end of the season to!
Second: defoliate your tomatoe plants. Yes, this means prune off all the leaves. Also remove any yellow flowers.
The plant will now invest all it's energy into ripening the fruit.
OH! If only it were that simple!
If the plants are indoors:
Clean the plants, but leave the yellow flowers. You may still get fruit from these.
(Zebra tomatoes are really slow to ripen (in my experience) so, try this defoliation technique and it will hurry their ripening process along.)