"First with regard to the knowledge of sense perception, whenever an object makes an impression on our normal sense organ by coming into contact with it, we may safely believe without any doubt that it is in reality as we perceived it, provided we are sufficiently expert not to be misled by illusions...." - Saadya Gaon
What does it mean to say that something is as we perceive it? This is a difficult issue, and I want to show just how difficult it is. Let's start off with the simplistic view: I am sitting in my chair and I see a red book in front of me. Therefore, barring hallucinations, colorblindness, and so on, the book is really red, in an objective sense.
However, it turns out that it is me who makes the book red. The different wavelengths in the world are analog and continuous - there is no nice division of colors "out there." Colors get divided up in our brains. The rods and cones in our eyes divvy things up according to, more or less, the three primary colors of light (red, green, and blue; some women have a fourth set as well). Another portion of the brain (the LGN) contrasts certain colors and is responsible for that after-image you get when you stare at something too long. This opposes blue and yellow, and red and green. Linguistic systems add colors in a specific order: languages with only two words for colors talk about white/light and black/dark, languages with three invariably add red as a third, and so on. Our brains make things the colors they are, so the book is subjectively red.
But even if our brains shape the world, there is still the world to be shaped. My seeing of the book as red is as dependent on my brain shaping the world as on the world shaping my brain. There are certain wavelengths which trigger certain neurons, and both wavelengths and neurons are necessary. So the book is red in this subject-object interaction.
Of course, things get even more complicated. On the one hand, I am as much a part of the physical world as the book. The book would probably appear as a different color under the light of a different sun; is it any less red for that? But I am as much a part of this physical environment as anything else. I am here seeing the book in this way because of specific pressures of natural selection, formed in relation to the world. So my own subjective view of the world is not so subjective - it formed because of the world, in tune with the world (or else the lineage leading up to me would not have survived). So maybe the book is objectively red after all, though in a less simplistic sense than we had first considered; perhaps it is better to say that it is "materially red", since there is no longer any separation between me the subject and the book as the object.
On the other hand, everything that I see in the world is a form of consciousness. Everything sensed, thought, inferred, is all a way in which I am conscious of it. The book's being red is a form of consciousness like anything else. Everything that I figure out about natural selection or the physical structure of the world is grounded first and foremost in consciousness, and makes no sense apart from it. However, it is not just me as the subject imposing a view on the world; the world as form of consciousness is just as important. When I see a tree, the tree is just as important to that event as me. In fact, it is after the fact that I separate out the aspect of consciousness which is "I" and which is the "tree", based on the fact that they move in different directions. So the book would be ideally red, or red as a structure of consciousness (just as "I" end up being merely a structure of consciousness), and not just subjectively red.
It would seem that we need at least one more level, however. We can start from the book and then bring ourselves in as an object of natural selection, or we can start from ourselves and bring in the book as a form of consciousness. Somehow, both of these work together. We need to talk about consciousness of the world in order to let science get off the ground in the first place, but science has something to say about our consciousness of the world. There is a circle here, and so the final ground I will talk about is the whatever-it-is that is the basis for this circle which allows it to take place. In order to figure out what this basis is, I give the simple homework assignment of reading Plotinus, Proclus, Nagarjuna, Vasubandhu, Dogen, Nishida, Shankara, Schelling, Hoelderlin, Hegel, and Heidegger, for a start.
At this point, since it all works together, subject and object, matter and consciousness, maybe it is best to simply say that the book is red.