He's trying to get to the bottom of reality, and separate it from perception. For example, the tree falling in the woods scenario: there's no doubt that the tree falls, and in so doing, it cracks branches and excites molecules, disturbs wildlife and has a real effect on its surrounding. But does it make a sound? It makes sound waves. But sound, by definition, must be perceived in order to be. So all he's doing is shaving away the perceptions, trying to find the basis for reality. It's the same with colors. They're just fragments of reflected light in a spectrum. They're only colors as seen through our eyes.