Sound like you are reading an awful lot into your experiments. LED's vary greatly in their output capabilities. Your requirement of using 2 red LEDs for 1 each of green and blue speaks more to the type of LED's you are using than the differences between colors. Decent RGB LED's are tuned to allow one to achieve a fairly convincing "white" with little difficulty.
The other thing is you will never generate the other end of the color spectrum which includes black(which is the absence of color). There is three colors that can generate black but these are not made to my knowledge at least not in a led. I know you're saying i thought he just said black was the absence of color but now we are talking about black as being generated as a artists color. If you are making you own lighting you can use tinted acylic(non glare) and put it over the white. Now the great mystery is how does a TV get black?
There are no "colors" that can generate black. Black is the total absorption or lack of light. Putting a tinted acrylic cover over a white LED does nothing more than filter out certain wavelengths relative to the specs of the material. If the wavelength isn't there in the LED, or not there in sufficient levels, the tinted cover won't do what you expect it to. If you are talking about dark tinted acrylic to produce "dark" light, then that is just silly.
There is no great mystery as to how "black" is generated on a CRT. The electron beam gun shuts off and the phosphor is not excited so no light is produced.. The quality of the "blacks" you get are based on the shade of the phosphor on the inside of the screen, the tint of the CRT glass, and the amount of ambient light in the room where the CRT is viewed.
Additive and Subtractive Color
Offset printing, digital printing, paints, plastics, fabric and photographic prints are based on the subtractive system of color (CMY/CMYK) in which cyan, magenta and yellow mix to form black (K).
This is just incorrect. (K) is black. No real offset / digital printers use a 3 color process, rather a 4 color process which includes real black ink. Cyan, Magenta and Yellow by themselves just produce a muddy dark gray, which is why inkjet printers which use separate CMY and Black (K) ink cartridges produce vastly superior text and imagery.
Also, this is not precision spectrometry here. If you are trying make an orange light, and of 10 people 9 say the color is "orange" and the tenth says it's a slightly reddish orange, there's good chance you succeeded in making an orange light. Anything beyond that in this application is overkill to the nth degree.
Presets with user defined R, G and B values which are linked to on-screen color icons is probably all anyone would really need.
RandyT