Yellow tomatoes are tomatoes which have been bred to be yellow, rather than the traditional red. In addition to yellow tomatoes, it is also possible to find green and purple tomatoes in some markets. Many of these tomato varieties are actually quite ancient, but were only widely introduced recently, as gardeners and consumers realized that this New World fruit comes in an assortment of colors, shapes, sizes, and even flavors.
The difference in color is not just superficial. The nutritional profile of yellow tomatoes is slightly different from that of red tomatoes. Yellow tomatoes have lots of niacin and folate, less vitamin C, and less lycopene than red tomatoes. Perhaps most importantly, yellow tomatoes are lower in acid than red tomatoes, and some companies have even developed almost acid-free yellow tomatoes.