The proof is in the fact that I have a litter of three day old yellow tri babies 3 red eyes, 5 black eyes. If the red eyes were from the c-locus dilution I'd expect them all to have red eyes. The siamese marked tris have red eyes because siamese have red eyes; with the tri factor, if it affected the eyes I'd expect the eyes to revert to black. The transgenic factor causes an individual with a dilution to revert to darker shades, not lighter shades. So when people talk about a beige brindle, it means that the mouse would have only beige on it, but the transgenic factor causes patches to revert to darker versions of that same pigment ( chocolate, coffee, black) that causes the original color, but is diluted by on the c-locus. The tri factor does not affect eye pigments.
And red/yellow also can come from A^vy, which are what all of mine that aren't ee come from. I took years ridding my A^vy line of brindling. Not that I don't like brindling, I have a line of brindles that I keep very, very separate, cuz once it gets in there, like I said, it takes years to get it back out.