What i am wondering is, why Cyan, Magenta, Yellow and not Blue, Green. Red
Is it easier to print color with a CMY head.?
Like koraks says, it's just different ways of doing essentially the same thing.
Let me explain the "subtractive" thing a bit. This is where you use a nominal "white" light source, then insert filters into that light path, essentially "subtracting" colors from it. (Note that I say a nominal white light; historically this has been from a tungsten (filament) light source which actually tends yellowish, for what it's worth.)
Looking at the paper, it has three dye layers, called cyan, magenta, and yellow. Which are sensitive, respectively, to reddish, greenish, and bluish light. Now, if you start out with white light and want to reduce the amount of reddish light, what do you do? You insert a cyan filter; this reduces the red light without affecting the amount of green or blue light. You might ask, is there any other filter that can reduce red, and only red? No, there is not. Likewise with greenish light; only a magenta filter will reduce the green, and only the green light. And on to bluish light; only a yellow filter will reduce the blue, and only the blue light.
So the bottom line is, that in order to directly control each of the paper's dye layers, individually sensitive to red, green, and blue light, you need to use cyan, magenta, and yellow filters in a light path.
Now, you originally asked about the alternative use of red, green, and blue filters. Can these be used instead? Sure, here's how it would work: say that you want remove some reddish light, what would you use? Well, it won't be a red filter; red filters let all the red light come through unhindered. (A red filter blocks all colors that are NOT red, meaning that it blocks both green and blue.) So to block red light, using RGB filters, we'd have to use both green AND blue filtration. But there are side effects. The green filter blocks every thing that is not green, meaning it blocks both red and blue. And the blue filter blocks everything that is not blue, meaning red and green. So we ARE able to successfully block red light by using both green and blue filtration. But... we also have to block some green and blue along with it. You can use a similar line of thinking to figure out the situation for trying to remove just green, or blue, light. They work in a similar way - always requiring two filters, and having a side effect on colors that you didn't want to affect.
So this is roughly the situation for a subtractive system, using a single light source. When you use the "subtractive primaries," cyan, magenta, and yellow filters, you can have direct control over the individual colors of the light, red, green, and blue, without side effects. Something else should also be said... this works only because the filters have only partial blocking power. Meaning that they modulate the amount of colored light. You could not do this with 100% hard-cutting filters, as they would completely block everything.
This whole thing might be hard to grasp if you're not already familiar with the filters. If you want to try to understand the mechanism you might try sketching a graph of the spectrum, labeled in wavelength. (We see roughly 400 to 700 nanometers, where roughly 400 to 500 is bluish, 500 to 600 is greenish, and 600 to 700cis reddish.) If you draw a high-up line across the top as the starting amount of white light, then you can sketch in the effect of different filters - that is, what is left when a colored filter removes certain wavelengths. I think this is about the only way to get a real understanding of the effect of filters. But probably not worth the mental effort for regular photographers or printers. Only if you find this sort of thing fun. Most workers in the field just learn some rules to follow, and this is generally good enough.
Hope this helps a bit. When I was a youngster no one could explain this sort of thing to me. So what I like to do here is to explain things the way the younger me would have wanted to know back then.
