How is the brain capable of seeing a spectrum of color when the eye only contains three types of color detectors?
By integrating them in the optical cortex. The three types of receptors peak at yellow, green and red and are thought to process the full field of color through opponent-process theory. For this theory to work, each color will activate and inhibit a given receptor, and having it activate the correct receptor will give an inhibitory response to the other receptors. Red excites the violet receptor but inhibits the green receptor, for example. This is how the eye sees red. Blue and yellow are opposites too. The brain integrates all the stimulatory and inhibitory impulses and mixes them together in order to determine what color you are looking at. Different strengths of receptor activation result in the perception of different colors as the brain combines and integrates the incoming signals from the retina. Colorblind people have a problem with the processing in that the cells in the brain that tell the difference between red and green are missing. There are cells in your brain called the parvocellular cells that process the differences in activation between the green/red receptors, and another type that processes the blue/yellow difference.