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In reply to Colette's post, I also wonder how much of what we physically perceive is selected by the brain.  I came across this article on ScienceDaily.com about a study conducted by two University of Delaware psychology professors (published in April 2010 issue of Emotion) about how jealousy can literally make someone blind.
I went on Tripod and tracked down the abstract:

Does the influence of close relationships pervade so deeply as to impact visual awareness? Results from two experiments involving heterosexual romantic couples suggest that they do. Female partners from each couple performed a rapid detection task where negative emotional distractors typically disrupt visual awareness of subsequent targets; at the same time, their male partners rated attractiveness first of landscapes, then of photos of other women. At the end of both experiments, the degree to which female partners indicated uneasiness about their male partner looking at and rating other women correlated significantly with the degree to which negative emotional distractors had disrupted their target perception during that time. This relationship was robust even when controlling for individual differences in baseline performance. Thus, emotions elicited by social contexts appear to wield power even at the level of perceptual processing.
Most, Steven, and Jean-Philippe Laurenceau. "Blind Jealousy? Romantic Insecurity Increases Emotion-Induced Failures of Visual Perception." Emotion 10.2 (2010). Web. 18 Apr. 2010.

This is interesting because now the phrase "you only see what you want to see" has even more weight.  Yes, the physical reality of the world consists of light, space, matter, etc. and color can be dismissed as a series of photons traveling on some set wavelength.  But what we see, what we actually see can easily be affected by what our brain dictates.  As the functions of our brain and the impulses, ideas, emotions, etc of our mind meld together, our behavior (sight/perception) must change accordingly. 

This question "if the brain imposes its own construction on perception, how much of what we actually see is real (versus "fake" or an "illusion")?" seems to be popular in this forum.  However, I'm beginning to develop a distaste for the label "illusion" or "fake."  I don't think any part of life, generally, is an illusion. People do have different perspectives and backgrounds that help create a range of lenses with which they can experience life.  Maybe in a vacuum, we would and could see the world as it truly is (color as a physical entity). But in this world - our world - where social interaction, social construction, cultural and societal bias is nearly unavoidable, we see color because our brain wills it or constructs it.

link to sciencedaily article.