My grade school in Spain had a color-coded system for test scores: A was red, B was blue, C was green and F was brown. So the color distribution in your academic chart revealed at a glance how well you were doing in your various classes. One result of this system was that red and brown became, respectively, my favorite and least favorite colors as a child.
This story is one of many examples of how an essentially neutral visual stimulus (the color red in this case) may become associated with a reward value (a good grade). From this information it is easy to predict that neurons in the brain's reward-processing system--a network of areas connecting the thinking cortex to the emotional centers in the brain's limbic system--may be activated by the physical properties of those sensory stimuli that come to be associated with rewards. We learn to associate certain stimuli with positive feedback; thus, my brain's reward centers learned to react happily to red.