Hi dhroane, let me see if I can explain a bit better.
Normally, we name a filter by what color it passes. For example, if we look at a white light, then raise a filter in front of our eyes and it appears to be red, we would call this a red filter. And of course, it looks red because only (or mainly) red light is passing through to our eyes.1)What does "filtering" connote?
In the case of "light" we cant's see, such as UV (ultraviolet) or IR (infrared), the naming is not always clear. So rather than say that something is an IR filter, it's better to call it either an IR-pass or an IR-blocking filter.
Well, as I said in my first post, in order to truly understand, I think it is necessary to look at the full spectral data. The following explanation, using words, is going to sound complicated and hard to follow, but it should work. ps; no one really needs to know this, it's only for intellectual curiousity, I think, or to make you feel more confident in the rules of "doing the wrong thing."To block magenta and yellow would assume that these two colors "mix" to create the red you are trying to block.
So in keeping with the simple concept, where white light is made up of reddish, greenish, and bluish light, here's as simple as I think I can exlain it. The primary filters, red, green, and blue, will pass only one color (their own), while blocking the other two. The secondary filters, cyan, magenta, and yellow are considered to pass two colors, while blocking only one. For example, a magenta filter will block green, but passes both red and blue (red and blue light, mixed, appear to us as magenta).
So, for your question about blocking magenta and yellow: to block magenta, as you now know, actually means to block both red and blue, while passing the third color, green. in a similar manner, to block yellow means to block both red and green, while passing blue. Clearly these two things are at odds with each other; the one passes only green, while the other passes only blue. If you combine both filters, it seems that nothing should come through. And this IS the case with perfect filters, where they completely pass or block. However, color printing filters are not in the class of all or nothing; they block certain percentages of light. For example a 30 cc yellow filter should block about 50% of the blue light, while allowing virtually ALL of the red and green to come through.
So lets redo the "blocking magenta and yellow" thing with 30CC filters. When we start with white light (100% red, 100% green, and 100% blue), then try to block magenta, this calls for a green filter. So after passing through our 30CC green filter, we'll find this: 50% of red, 100% of green, and 50% of blue light. The next step, blocking yellow light, calls for a blue filter. So applying a 30CC blue filter to the now greenish light, here's what happens: blue light will pass through unaffected, but only 50% of the existing red and green light gets through. Consequently we'll end up with: 25% of red, 50% of green, and 50% of blue. So the final result is that green and blue have equal strengths, while the red has been weakened to half of them.
The net effect is, that red light has been relatively weakened, giving the visual appearance of greenish-blue light, which we might also call cyan. So you can see that things DID behave correctly, our initial intent was to (relatively) block red light, and this was the actual result, although we did it by trying to block magenta and yellow light.
Once you are satisfied that things work, it is much easier to simply draw the little color diagram, either the triangle thing or the overlapping circles, and then follow the rule of "always do the wrong thing."