so-called white tomatoes are typically light yellow. As for kumato, I think they're still a hybrid and not open pollinated. The saved seed
could produce almost anything.
While tina is right that crosses can occur, with tomatoes, they tend to be fairly reliably self-pollinated, with the occasional bee pollination resulting in crosses. Some growers don't isolate plants or bag blossoms, or hand pollinate, and the saved seed
is roughly 95% true to type. That is the case with OP or open-pollinated tomatoes, often referred to as "heirloom" whether they are heirlooms or not, (the definition of heirloom is somewhat contentious,) the process of developing an OP tomato from a hybrid takes roughly 7 gnerations depending on the number of plants grown out from the seeds
of the original hybrid and recognizing desired characteristics that indicate certain stable traits...anyway, as for identifying a certain tomato goes, it's easier to do when a variety is from an OP and the possible parent can be narrowed down, hybrid offspring or cross-pollinated, accidental hybrids aren't going to be easily recognized.