Ok, that makes total sense then. The red grape is definitely where the color comes from. Knowing that, I would guess that the change in color is mostly due to the sediment dropping out. Keep in mind yeast is pretty light in color (usually an off white) so when that dies off and falls out of the mixture, it's going to get much darker.
I did 2 different batches, one from red tomatoes and one from orange tomatoes. Mine were very similar to yours, minus the grape concentrate. Also I used acid blend instead of lemon juice. So far I like the orange tomato more than the red. It tastes like a pinot grigio with a tomato aftertaste. People are really surprised with how good it tastes. The red tomato is also good, but the tomato taste is much more potent. The orange tomatoes that I used were these little cherry sized tomatoes. They were super sweet and not very acidic. I'm thinking next year I'm going to grow a bunch of them and make a big batch. The 2 on the left are the tomato wines.
I don't think the red grape concentrate was necessarily a bad idea, only time will tell. Let us know how it comes out!