Are colors natural and inherent properties of physical objects? Or does the viewer determine, through the act of looking, the color of an object. Are the colors we see just an illusion manifested in our brains? I do not know the answer to these questions and they are questions that have been asked for ages. In some respects, I think it's a waste of time to ask these questions. A lot of our lives are dependent on our perceiving them and if we were able to answer this question, it would only raise many other questions and leave us just as confused. But there are many fun and interesting color illusion websites that can get you thinking on the topic. Such as this website: http://www.psy.ritsumei.ac.jp/~akitaoka/color-e.html.
The Illusion Theory of Color puts forth that in our experience, objects have colors, but in reality, color is not an inherent property in the nature of the object. The sky and ocean appear to be blue, but they are not blue in the way we understand it. Some theories say the ocean is blue because it is reflecting the sky, but the sky is not really blue at all, that is just the way we see it. When the sun's rays enter our atmosphere, oxygen and nitrogen scatter the rays on the shorter end of the wavelength the most (violet and blue). Since our eyes are more sensitive to blue, that is how we see it.
If objects don't really possess color, then why do we see them? I think color is a human adaptation that allows us to differentiate areas of interest and danger. Don't eat those yellow berries, eat these nice red ones! Although, the berries might not be red, just the way our brain perceives them, it is irrevelent if it can keep you alive. The strongest will is the one to survive, and it doesn't matter where information comes from as long as it's useful.


Sources: http://www.why-is-the-sky-blue.tv/why-is-the-sky-blue.htm
http://plato.stanford.edu/entries/color/#Ill