To beg the question (which I think I know the answer to), in this context, how do I trust a camera, or my eyes, to give me a true result?
"Green" or other color words are already in your vocabulary of trusted primitives, otherwise the question couldn't even be spoken. "Green" is (to a good enough approximation) a particular pattern of activation of cone cells in your retina, or (an approximation of an approximation, but still good enough) a point in the color space computed by a camera's CCD photoreceptors.
The query at hand is whether the same cone cells are firing (or whether the camera's CCDs return the same value) when looking straight up when out of doors on a cloudle...
Let's say that I believe that the sky is green.
1) How can I know whether this belief is true?
2) How can I assign a probability to it to test its degree of truthfulness?
3) How can I update this belief?
Thank you.