Colourful in Theory - That Irks Me

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Normal colour displays have 3 differently-coloured sub-pixels (red, green and blue). What happens if we add a yellow one into the mix?
Corrections:

4:05 : It's actually called a "Chromaticity" Diagram, not a "Chromacity" Diagram.

4:39 : As Antsaboy94 pointed out, there technically can be formats that would be more accurately displayed with new subpixels of certain colors, since there exist color space standards (such as Adobe RBG) which support more colours than a conventional display.
Sources:

Colour Vision
http://www.handprint.com/HP/WCL/color1.html

Cone Fundamentals
http://www.cvrl.org/cones.htm

Color: the 1931 CIE color-matching functions and chromaticity chart
http://rip94550.wordpress.com/2009/10/18/color-the-1931-cie-color-matching-functions-and-chromaticity-chart/

Music in this video (downloaded from the YouTube audio library):
The Messenger
The Story Unfolds

Видео Colourful in Theory - That Irks Me канала Frame of Essence