The CIE spectral RGB discussed before can also be used as the bases of the
color space. Each color is a point in the space with the coordinates
representing the amounts of the RGB primaries needed to match the color.
We see some colors have negative Red coordinates as they need to be matched
with negative Red primary. Specifically, the spectral colors in the range of
are on the negative side of red axis, as
indicated by the intersections of the spectrum locus with the vertical (green)
axis. In other words, only the colors in the positive octant can be physically
produced by mixing these three primaries.
