Dorine, I will say there is one other possibility for you tomatoes color change that I will let you know about. I don't know how far away from any other garden you are like a neighbors or something to where a little bee could have crossed something. Where I live we have little sweat bees that like to gather pollen for some reason that have unknowingly crossed their share of plants. Even just one grain of pollen would have been enough to change the outcome of the one seed you planted. Many don't realize but for every seed that you find in a tomato and many other fruit, it takes one grain of pollen to create one seed so as to reproduce. So lets say you have a good seedy tomato with 200 seeds. 200 grains of pollen had to get on the stigma and travel down to the ovary and develop. With that in mind, if we were to insert 200 different grains of different varieties, the resulting tomato would have the potential to create 200 totally different offspring. Now in the real world that could never happen, but it goes to show what one microscopic grain of pollen can do. But to find out if it is a true sport, they will need planted again. if they come true in every respect to what they were before, it was a sport.
Just a little more fuel for garden thought. Mark