Tomatoes are tropical but there are so many varieties that have been grown across the globe, you may have a variety that yellows out in your climate zone. High heat 95+ degrees can cause different varieties of tomatoes to yellow. This is normal. After you grow a variety of tomato for a season, you have to decide if it can handle your climate. I show case the 'Brandywine' that always yellows come high heat... but I keep trying. Join My Google+ Gardening Community called Our Tomato & Vegetable Gardens - we are approaching 2500 world-wide gardeners: https://plus.google.com/communities/114956817444053979636 or Link from My YouTube Page.