What we perceive is an interplay between what a part of what is present to our senses and ouselves. In the colour example, an object reflects some light having a certain distribution of wavelengths, our neural apparatus interprets the ratios between those wavelenght as "greeness" and we think of that the object as being green. If it weren't for the light source (assumed white in this case, with a red light the object would be interpretd as black), the object, and our perceptive/interpretive apparatus, that greeness wouldn't be there. So instead of thinking of ourselves as being separate from the world we observe, or that the world is entirely in our heads, it seems that the way we see and interpret the world is part of the world.