I'm currently reading Color Design Workbook: A Real World Guide to Using Color in Graphic Design, but it's a bit short on color theory basics. I was wondering: in CMYK, black is added to the three subtractive primary colors to improve the rendering of blacks, which would otherwise be brownish. This addition of a fourth pigment increases the gamuth of available color.
So, my question is: has there been any serious proposal to add a fourth primary to the additive RGB colors? One can imagine that this would allow for more vibrant non-primary colors, for example in the “violet” region, in high-end display devices. One obvious downside would be the price of such new technology, and I know it is not widely available, but: has it been considered?