I just want to reiterate what asparisi said above, because it's an awesome point. "The sky is blue" is just a short-hand for "my brain converts this specific wavelength that enters into the eyes into an experience of blue". And then you realize that most (or even all) observations are of this type, they start with a subjective experience. Luckily these subjective experiences are more or less persistent over time and across humans, so we assume a shared persistent "reality".