There are two types of green tomatoes to consider. One is simply an ordinary tomato that was harvested before it was fully rip and the other is the one that’s actually green when it’s fully matured. The second one is rarer and it’s an heirloom variety.
The best way to tell them apart is because the real green tomato often has vertical stripes or other variations in the coloring, will feel soft when pressed, and will taste much like a red tomato, possibly slightly sweet or spicy depending on the variety. The ordinary one will be firm and more bitter.