But what would count as a testable experiment to prove the existence of colors? Remember, at the time, there was nothing other than human vision that could measure them. And we had no idea what green actually was, other than that people said grass looked green to them.
The situation with natural rights is currently about the same as it was then for color. So what would have convinced you back then that colors actually exist?
If you want, you can assume that you lack color vision. Because even though you don't lack the ability to sense natural rights, you will probably stubbornly insist that you cannot sense them and the fact that almost everyone else agrees that torturing children for pleasure is wrong is just a mysterious coincidence. All I can say to that is what I would say to the person who insists a green cube and a red cube look the same -- you are either lying or you lack a sense the rest of us have.
There is one huge difference.
People in 1012 saw the same green that we do. We know this from their literature.
They didn't see the same natural rights that we do. Their natural rights included the right to own slaves.
So the use of colour analogy doesn't work.