It's kind of simple: Does it seem that the sky is blue? Then accept it as a temporary truth and move on. Learn about physics, colors, human brain, et cetera. Every Friday review your beliefs about the color of the sky, and update them based on your current knowledge.
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.