Diclaimer: I have full-color vision, but with color blindness there are (roughly) two dimensions of color, whereas most people have three (and a black and white photograph has one). What this means is that with 1D color, you can sort all of the colors you see into a line -- dark to light. With 2D color, you can sort all of the colors you see on a flat plane. With 3D color, you require stacking colors.
Now, there's plenty of animals out there that have more than 3 dimensional color (they have more than 3 types of cone cells). So two colors that look the same for a normal person will look completely different for an animal. Some octopi have eleven dimensions of color! To them almost every human would be severely colorblind.
For most people, yellow photons striking their retina will produce the same effect as a combination of red and green ones. If you look at this cone cell response graph (http://upload.wikimedia.org/wikipedia/commons/1/1e/Cones_SMJ...) you will see this is because 520 nm + 640 nm light can give the same effect as 580 nm light with the appropriate intensities.
Having 3 cone cells is the reason that most electronic equipment has 3 types of subpixels (red, green, and blue). If you take the colors of these three subpixels, locate them on the CIE 1931 chromaticity diagram (http://upload.wikimedia.org/wikipedia/commons/b/b0/CIExy1931...) and draw a triangle to connect the points, this triangle is the color gamut of the monitor. The more types of subpixels you add (or the further apart they are in color), the more colors your monitor can reproduce. This is why Sharp's television with that 4th yellow pixel can reproduce more colors that humans can see.
It's thought that some people might be tetrachromats. They have 4 types of cone cells because of a genetic mutation. There's still some questions on how this extra information is processed by the brain, but there's a chance that for these people almost everyone else seems colorblind. They are able to distinguish two colors that everyone else cannot. This also means that television won't reproduce colors correctly for them, and it won't look natural.
That's interesting about tetrachromats. I knew a lady when I was a kid who claimed she couldn't 'see' an image on TV, that it was all just garbage to her. I'd always just assumed she was either crazy or making some weird kind of statement. Maybe not!
That would have to be something else. The effect of an extra cone on watching TV should be far less severe than a normal eye's view of black and white.
Now, there's plenty of animals out there that have more than 3 dimensional color (they have more than 3 types of cone cells). So two colors that look the same for a normal person will look completely different for an animal. Some octopi have eleven dimensions of color! To them almost every human would be severely colorblind.
For most people, yellow photons striking their retina will produce the same effect as a combination of red and green ones. If you look at this cone cell response graph (http://upload.wikimedia.org/wikipedia/commons/1/1e/Cones_SMJ...) you will see this is because 520 nm + 640 nm light can give the same effect as 580 nm light with the appropriate intensities.
Having 3 cone cells is the reason that most electronic equipment has 3 types of subpixels (red, green, and blue). If you take the colors of these three subpixels, locate them on the CIE 1931 chromaticity diagram (http://upload.wikimedia.org/wikipedia/commons/b/b0/CIExy1931...) and draw a triangle to connect the points, this triangle is the color gamut of the monitor. The more types of subpixels you add (or the further apart they are in color), the more colors your monitor can reproduce. This is why Sharp's television with that 4th yellow pixel can reproduce more colors that humans can see.
It's thought that some people might be tetrachromats. They have 4 types of cone cells because of a genetic mutation. There's still some questions on how this extra information is processed by the brain, but there's a chance that for these people almost everyone else seems colorblind. They are able to distinguish two colors that everyone else cannot. This also means that television won't reproduce colors correctly for them, and it won't look natural.