Look at this image (http://upload.wikimedia.org/wikipedia/commons/thumb/6/60/CIE1931xy_CIERGB.svg/325px-CIE1931xy_CIERGB.svg.png http://upload.wikimedia.org/wik

Look at this image (http://upload.wikimedia.org/wikipedia/commons/thumb/6/60/CIE1931xy_CIERGB.svg/325px-CIE1931xy_CIERGB.svg.png http://upload.wikimedia.org/wikipedia/commons/thumb/6/60/CIE...) from Wikipedia. It represents all the colors (chromaticities) that humans can see. A device that reproduces images using primary colors can reproduce all the colors within the vertices of the primaries used. For example, an RGB monitor can reproduce all colors within the triangle formed by the particular R, G, and B subpixel primaries as plotted on that diagram. What you'll notice though is that because the diagram has a curved edge (this is the spectrum of visible light actually), no polygon can encompass all colors. I've always thought it would be cool if someone made a CRT using two prisms (splitting light from a blackbody), such that a narrowband of light from one prism is combined with a narrowband of light from the other prism in different intensities. This would be able to reproduce all colors a human can see, assuming the ability to tune to a specific portion of the spectrum is fast enough to scan over all pixels that constitute the image.