I noticed the juubi has one eye that has several ripples with sharigans along these ripples. Now i start to think is this it's real eyes or a projection of the so6p's eyes. However for all the images we have seen of the juubi, thats the eye they place so this gets me and i assume, that has got to be it's real eye.
This is my opinion, the so6p is a senju because i think he gained the optical abilities after he sealed the juubi into his body. The act must have activated his eyes before he had his first son.

These are the facts to support my claim:
-the so6p preached peace and ninjutsu, some thing not many uchiha would do. He wished peace and left his will in the hands of his younger son whose ancestors came to be called senju.
-His eyes only have the ripple effect in the rinnegan, they don't come with the sharigan part in the juubi's eye(the few images we have been shown prove this).
-Most uchiha are sinister in nature, just like bijuus and have the ability to manipulate bijuu with ease. l think it's easy for them because they have a DNA link to the juubi than the senju who can only tame the bijuus and can't manipulate them.
-The uchiha don't usually practice sealing jutsus or seal bijuus into their bodies like the senju do. They simply use them as tools so the s06p is more likely to be a senju because he practices sealing jutsus.
-They are born free of senju DNA so where else are they getting this massive chakra from apart from there connection to the juubi.
l conclude by claiming the uchiha are more related to the juubi than to the senju like the stories about the s06p's sons claim.

I will point out right now that my theory is not water tight because am bothered by how the s06p was able to use abilities like izaragi to split the juubi into 9 bijuus if his eyes never switched from a rinnegan to a sharigan like we notice with madara.
Thats my opinion.